It will be good to create 10 such query activities in your workflow, one each for
- Family member 1
- Family member 2
and so on.
Check the corresponding birthday against the 14 days criteria.
Connect the outgoing transition of all such query activities using a Union.
The only concern I see with this logic is, if two members of the same family have birthday on the same day, this will send an email to only one of them.
Regards,
Vipul