So my ex husband and I have spilt custody of our 2 children.. on his days the kids stay at his moms house.. he recently has gone to prison for dui manslaughter and the kids have been staying at his parents house for 4 months because I hit hard times I still see them on the weekends and now have a stable home for them and am wanting them back full time.. my question is do his parents have any rights or can they fight me for them while he is in prison for 2 years?

They have no custody rights, they may sue for a grandparents visits ( but you should be allowing some visits)
Remember after 4 months the may be confused at the move and you need to work at it, also understand grandparents concerns and missing the kids. They were there for the months you needed them, so don't do them wrong in this either