It sounds like your aunt is elderly so she might not be on Facebook, although you might be surprised.  But her children would be in their maybe 40s or 50s so would more likely be on Facebook.

Be prepared for things to be a little stilted at first.  They got pushed away by the  JW mentality and might be uncomfortable opening up.  Give it time . . . I've just recently reconnected with my mom's family and it's gotten better and better.  And, it feels so good to have a family again.  It's worth the effort.

Good luck on you search.

Sandi