Is a 17 month old too old to be nursing?

My sister in law still nurses her 17 month old daughter quite often during the day. I do see the child eat gerber graduates pick-up food and #2 and #3 baby food also given to her at lunch time. Usually breakfast and snacks are all nursing. The girl has a mouth full of teeth and we are a little concerned. We have brought the subject up tactfully and we get shot down defensively, lol. She says, "She's MY child and I'll nurse her as long as I want to." The weird thing is that my neice walks up to her now and asks for numb-numbs while pulling at her shirt.