We bought Christmas stockings for each of our in-law children long before they were married. In fact, my son had only been dating
his (now) wife for about three months before their first Christmas here.
We filled her special stocking with several small, moderately priced items and we told her she could take her stocking home with her, but that if she came back next year she'd need to bring the stocking back with her because our Santa only supplies one stocking per person.
She brought it back over Easter to make sure it would be here for her -- her way of letting us know she did indeed intend to be back sharing future Christmases with us again.
Basically we did the same thing with our sons in law. We still have all the original stockings!
OP, I can't imagine what it would feel like to be your future sister-in-law and sit around watching everyone else opening presents knowing that there would be absolutely none for me because I wasn't quite yet "one of them'. I think it would be very hurtful.
Unless that's what her parents do to your brother too.