Would you bother??

I would stop. You've done it all her life and it's time she learned that politeness gets rewarded .She wishes to ignore you, so do the same to her, it sounds harsh but my goodness at that age I'd have been over the moon if an Aunt and Uncle gave me money.Just stop it.

Unless you have an online account, I think it is not that easy to put money into another’s account nowdays. Certainly not cash.

If your niece doesn't only not thank you for gifts , but ignores you as well - start ignoring her .
She needs to learn some manners !

Don't send any more. It should not need her father her to prompt her to say thank you. I don't give to one of my grandson now24 , he's never thanked me for the past four years

I continued until 18, then another gift on their 21st, then stopped.

What does your OH think about it?

Don't give her anymore gifts. A thank you costs nothing! She needs to learn a lesson !

My Mum was an excellent knitter. When my brother and his wife were expecting their son she thought she would knit him something. They are very fussy people and we discussed this so she went to a shop and bought modern patterns and beautiful yarn by Debbie Bliss that would not disappoint.

She knitted a lovely jumper, a hat and some tiny socks. Next time I saw her I asked how the gifts were received. She was really upset and told me his wife barely looked at them, put them to one side and said “we only dress X in designer clothes.” To the day she died she just tolerated her and no more and I didn’t blame her one bit.

designer clothes Pah!
Do wish some people would get over themselves.
In the meanwhile give em nowt but contempt it is only what they deserve imo.

Don't bother, the ungratfeul little madam