It is interesting to me that everyone is talking about how the niece not writing a thank you note is the SIL's responsibility. I don't know about your SIL, but in my world (largely because my otherwise wonderful husband has a big end of fiscal year crush) Christmas looks like this: I work, do all the shopping, do all the cooking, do all the cleaning, do all the wrapping, keep the little people busy during their Christmas break, make the cookies with said little people, deliver them to older relatives and friends who do not get much Christmas joy, host out of town family (including my siblings and my siblings in law), organize parent gifts (for said siblings and siblings in law), and act as gracious host. I am happy to do all of these things, but sometimes it leaves me feeling like the set and clean up crew of a Broadway production. Post-Christmas, if someone gets side-ways about thank you notes, I'm going to blast them into next week. We tend to do a Christmas follow up at Valentines Day when we send out homemade Valentines to our extended family who sent gifts. I know, that is not Emily Post's recommendation, but Emily Post had time to hand write party invites, too.
In short, maybe you can make progress with your brother? A "hey, do you remember when mom made us do this as kids? It would really mean a lot to me if you helped your kids do this, too?"
Alternatively, set up something else with your niece, such as having her open the present while you Skype, particularly before or after Christmas, so that you can witness the opening and the appreciation first hand. Kids are always looking for an opportunity to open something "early" and I have found that my kids and my extended family love to skype with them just in general because the kids are so enthusiastic about it.
Good luck... and cut your SIL some slack and maybe get your brother to pick up some.