This is your family so I'm assuming you're somewhat close. I think its appropriate to ask what their intentions are to celebrate holidays. Will they celebrate one or the other or both? Or neither? Just let her know you'd like to support her wishes.
I like giving ornaments with a note yearly. Its a tradition my cousin started for my kids and now that she's gone, those notes and ornaments are everything to us. But that may not work if they are only celebrating Hanukkah.
Otherwise, books, a soft toy and diapers are my go to baby gifts.
dd1 10/05
dd2 11/09
and ... a mini poodle!