Who is a Christian according to the Russian Orthodox Church

As Russian Orthodox I'll try to answer your question. First of all, let me quote Saint Apostle Paul:

But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed. (Gal 1:8-9)

Also (in the same book):

Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God. (Gal 5:19-21)

So, according to the Saint Apostle(or rather according to God, as He spoke through the Apostle), there is only one true gospel. And a heresy is a mortal sin that makes its bearer lose the kingdom of God. Thereafter only one of Christian confessions is the right one. The detailed explanation of why we think Orthodoxy is that confession is quote long and goes out of the original question limits.

So to answer the questions:

Is it true that members of non-Orthodox churches are not Christians according to the Russian Orthodox Doctrine?

Probably they could be called Christians as it is generally accepted naming. But they arent getting Salvation, because they follow the wrong gospel.

Is it a sin to call them Christians according to Russian Orthodox doctrine?

No. But it would be definitely sin for an Orthodox Christian to tell a Non-Orthodox Christian that he may get the Salvation, staying in his confession. The sin of lie.

Sorry for bad English.

Update. I'll reference the Saint John Chrysostom to demonstrate that the Orthodox Church(which Russian Orthodox Church is part of) has literal understanding of above (Gal 1:8-9). The Orthodoxy counts The Saint John among the Three Holy Hierarchs(together with Basil the Great and Gregory of Nazianzus). His view almost always expresses the view of the Orthodox Chuch. And that is how he explains (Gal 1:8-9):

...And he(Saint Apostle Paul) says not, if they preach a contrary Gospel, or subvert the whole of the true one, let them be anathema; but, if they even slightly vary, or incidentally disturb, my doctrine... http://www.newadvent.org/fathers/23101.htm

First things first, there are no 'denominations' within Orthodoxy. The Orthodox believe in one, holy, catholic, and apostolic Church (as professed by the Nicene Creed), and they believe that the Orthodox Church is it. Therefore it would be inappropriate to speak of a doctrinal position of the Russian Orthodox that is not also true of other Orthodox. While they are under different jurisdictions, they are the same Church and thus share the same beliefs. With that said, there are at times differences in emphasis between jurisdictions, and they don't always see eye to eye. The difference is that they generally do not split from one another and form separate 'denominations' as in Protestantism. They remain in the Orthodox Church and continue to seek reconciliation. Orthodox recognize that reconciliation may take years, perhaps centuries. So they don't rush these sorts of things. I should also mention that it is very precarious to attempt to speak on behalf of all of Orthodoxy when it comes to this issue, but I will do my best to represent what is the most prevalent view within the Church, with the caveat that not all Orthodox may agree (and that is OK).

With that being said, the answer to this question depends on whether you are talking about the Russian Orthodox Church Outside Russia (ROCOR) or the Russian Orthodox Church itself. There shouldn't really be any distinction since as of 2007 the ROCOR is now in communion with all of mainstream Orthodoxy because of its incorporation into the Moscow Patriarchate. However, ROCOR is still self-governing, and its clergy and members are notoriously hyper-conservative and separatist. Regardless of this, they do not represent all of Orthodoxy if they claim that those who are not chrismated in the Orthodox Church are damned.

Fr. Damick offers a short explanation:

The final boundaries of the Church are known only to God himself, but outside the historical context of the Church—that is, the Orthodox Church—the nature of the connection of any human being to the Church (whether a believer in Christ or not) is unknown to us. Throughout Church history, various groups have broken from the Church, a tragic reality which does not divide the Church but rather divides believers from the Church. The final status of Christians in such communities is dependent on God’s mercy and grace, which is also true for those with membership in the Church in this life.

In this life, however, to be an Orthodox Christian means belonging to the Orthodox Church. It is not something one can do alone or as part of a separate group. Orthodox Christians believe that other Christian or even non-Christian groups may manifest varying degrees of the truth of the Gospel but that the fullness of the Christian faith is found only in Orthodoxy.

This is in stark contrast to Protestantism which considers the Church to be spiritual (and thus no one group can claim to be the Church). Orthodoxy, on the other hand, insists that it is "the fullness of The Church." According to Timothy Copple,

When the Orthodox Church says that it is "The Church," they are making no pronouncement upon the salvation of anyone inside or outside membership in Orthodoxy. This may be hard for Protestants to grasp since being saved and being part of The Church is practically synonymous when linked to the spiritual Church. The knowledge that not everyone, let's say, in the Baptist Churches will be saved only serves to reinforce the fact that the Baptist Church cannot say it is "The Church". Yet, they also firmly believe that there are many who will be saved, so neither can one say that any other group is "The Church."

While Orthodoxy does believe that ultimately to be saved means being in the Church and those outside the Church will not be saved, that issue is not fully decided until judgment day. Because salvation is not looked at within Orthodoxy as either an in or out position but a journey into God. We readily recognize that anyone inside or outside the Church at any particular point in time can be in the currents of salvation or not participating in it. Thus, there is no ability to point to any one person either inside or outside the visible Church and say they are saved or not saved. Whether any one particular person is going to make it to heaven we leave in God's hands. We cannot know the heart of the person, much less the disposition of God towards a particular individual short of God revealing that to us.

Orthodoxy also does not say that the visible governing body of hierarchs and the organizations that are called the Orthodox Church are in and of themselves "The Church". This is an understandable confusion because what is generally labeled as synonymous with the visible church in Protestant circles, if they have any concept of that, is the governing body, the denomination or local church. It is by becoming a member of such-and-such group that one attaches themselves with like-minded Christians and is called "a church" in a visible aspect. Therefore, when a group says it is "The Church", Protestants will tend to think that the group is claiming that their fellowship, their organization, their denomination or local church body is a one-to-one equivalent to all those names written down in the Book of Life.

Given what we just discussed above, it should be evident that this is not the case within Orthodoxy.

There are a lot of underlying worldview issues in this question that Copple addresses if you'd like to learn more (especially the tendency to separate the spiritual from its physical manifestation, a common dualistic worldview inherent in Modernity).

In conclusion, the general attitude of all Orthodox can be summarized by saying, "We know where the Orthodox Church is, but we do not know where it is not."