Completely ignored

by Tired of trying
(York, PA)

I have been in a relationship and living with my fiancé for the past 5 years. During that time I have repeatedly tried to have a relationship with his adult children, 27 and 22 years currently. Every year I buy them Christmas gifts and the children as well. The son acknowledges me when necessary and the daughter never acknowledges me. As a matter of fact she tells the son and everyone else around her that she thinks I am a know it all. She has asked my advice on certain subjects to which I gave my advice. I have never one time been anything other than nice, polite and never pry or to get between her or her father. She accused me and my daughters of stealing a video game system (which belonged to me) when her sister (half sister) had it. There are never any gifts for anything, never any cards, phone calls, texts, emails or facebook acknowledgements for any occasion from her or the son. They don't even acknowledge my children. Our income is limited. I really don't feel like kissing rear end anymore and buying them gifts of any kind. Of course I would love to continue buying their children gifts.
What is your advice?