I don't want this to affect my daughter, what should I do?

My HB 23 and his sister 20 have been fighting for over a year. She still lives w her parents so when we go visit for our daughter to see her grandparents, his sister is there. When we go out with them, she's there. My DD is now 19 months old and is starting to notice that her aunt is intentionally ignoring her (which she has been doing for over a year). My DD tries to get her attention and she ignores her to her face, walks away and plays with the other kids.