score:3
The document specifies that it is a declaration of both or one of those, being "garanzia" an economic statement and "alloggio" an accommodation statement. It is unclear to me why your brother needs this letter, but I'm supposing it is for his girlfriend.
Anyway, if the document does not say anything about those invited able to financially guarantee themselves (in fact, there are only options for you to guarantee for them - with your money), so whatever document they will attach, has nothing to do with this form. However, the accommodation proof should be enough, and if you really want to feel sure, you could let your brother wire you some money beforehand, and check the "I will cover his/her expenses" box.
"alloggio" is not "residenza". You are not being asked to provide the address you are legally bound to, but simply a place where you are able to accommodate your brother and his girlfriend. However, you still have to warn the local police that the people you invited over have come and are currently in their jurisdiction.