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Yes, Article 10 (Personal scope) of Part II (Citizens' Rights) of the Withdrawal Agreement:
(b) United Kingdom nationals who exercised their right to reside in a Member State in accordance with Union law before the end of the transition period and continue to reside there thereafter;
also includes the family members.
This has been clarified within the Questions and Answers statement of the European Commission published on the 24th of January 2020:
Who is protected by the Withdrawal Agreement?
The Withdrawal Agreement protects those EU citizens residing in the United Kingdom, and UK nationals residing in one of the 27 EU Member States at the end of the transition period, where such residence is in accordance with EU law on free movement.
The Withdrawal Agreement also protects the family members that are granted rights under EU law (current spouses and registered partners, parents, grandparents, children, grandchildren and a person in an existing durable relationship), who do not yet live in the same host state as the Union citizen or the UK national, to join them in the future.
Children will be protected by the Withdrawal Agreement, wherever they are born before or after the United Kingdom's withdrawal, or whether they are born inside or outside the host state where the EU citizen or the UK national resides. The only exception foreseen concerns children born after the United Kingdom's withdrawal and for which a parent not covered by the Withdrawal Agreement has sole custody under the applicable family law.
As regards social security, the Withdrawal Agreement protects all those EU citizens who, at the end of the transition period, were in a situation involving both the United Kingdom and a Member State at a time. Their family members and their survivors are also protected.
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