graduation gift for nephew when my sister spoke badly of me

My sister stopped speaking to me two years ago. The details are lengthy and childish on her behalf. Since then she has told our parents that I never treated her children well. She has three children and I have none. I took two on an all expense paid cruise, always gave birthday and holiday gifts, gave to school fundraisers, got one son a job at the firm I worked, etc. I know her statement is ridiculous but it hurt very badly. Now she has sent an invitation to my nephew's graduation. I'm at a loss. For someone who has never treated her children well and doesn't speak to me, what am I expected to do?