It is your SIL's loss.
I have a former SIL who, along with my former MIL, never paid any attention to my daughter after the divorce. I will spare the long story, but I finally told my SIL what I thought last week - and my daughter is 30. My daughter thought it was funny - she just doesn't care about that side of the family. And believe it or not, my SIL apologized. Said I was right, they had not treated my daughter as they should have. Go figure.