My sister says that sometimes she can tell the future.is this a delusion?

Don't pathologize: I personally doubt it is possible, but many perfectly sane people believe things that other perfectly sane people don't. Factually, you possibly could use the delusion word - but it's possibly misleading. If there are no (or few) other (unlikely-to-be)true things in her belief system - I'd avoid the provocative term. Also, w/ particular things we know a lot about, many of us can predict very well.

Probably: What does she mean? How far in the future? How far away? Does she tell you this only after the fact? Ask her to be sure to tell you every time she has a future prediction to write it down. Then compare after a while.