IDK, I think it's kind of punishing to give a gift with expectations attached. (yes, even when the expectation is Manners - we cannot control others, right?)
I think that if all this is hard for you to wrap your brain around sometimes, your niece must have equally big moments of confusion about her "loyalties" & the dynamics at play, regardless of her age. Maybe she feels torn being acknowledged while other people she loves are on the outs with you & simply doesn't have the maturity to express her feelings.
Quite frankly, regardless of whether we like it or not, thank you notes have become quite passe in this day & age of technology, outside of large events like weddings, baby showers, etc, Can you all honestly say that you send notes for every gift received, from every person, all year long? I don't know a single person IRL that does this for birthdays, holidays, etc. & I don't think twice about it because they'll have expressed their gratitude when I gave the gift to them directly - and if not, whatevs. Not my monkey.
Thing is - YOU decided to cut contact & pick & choose how YOU continue those relationships. They aren't required to comply with how you expect them to manage it, IMO.
If niece & nephew are grown & I feel slighted by her, I think I'm perfectly within my rights to stop sending gifts. But I also have to acknowledge that relationships are more than gifts & thank you notes - am I doing my part to be in her life? What's reasonable to expect in this type of relationship situation?