My sister has been living on her deceased husbands parents property for about 8 years. She has been taking care of the property, bills and paying taxes for as many years. Her sister inlaw came to the house and ordered her out. Does she have any rights or does she have to just leave?

If there is nothing in writing between her and her former in-laws then she is considered to be a month-to-month tenant. Depending on the location of the property her in-laws have to give her at least 30 or 60 days notice that her tenancy is terminated. If she hasn't left within the notice period then they can file a lawsuit to evict her.

There is no such thing as "squatter's rights" except under extreme circumstance, typically involving land with misplaced fences or incorrect land surveys. But I don't understand how the sister-in-law has obtained ownership of this property. Did the deceased husband have a will that bequeathed the property to his sister rather than his wife? Or was it titled in such a way as for ownership to go to the surviving sibling? Also what state is this in - is it a community property state?

Assuming for a moment that this is all correct and the property does now in fact belong to the sister-in-law: your sister is considered to be a tenant. In order to evict her the sister-in-law must follow the legal guidelines for eviction, which is typically a 30 day process for cases where there is no lease agreement. I would suggest that rather than force the issue your sister should reach an agreement with the sister-in-law for a definite time when she will be out. And she should have course stop paying any real estate taxes or repair bills immediately.

My sister has been living on her deceased husbands parents property for about 8 years. She has been taking care of the property, bills and paying taxes for as many years. Her sister inlaw came to the house and ordered her out. Does she have any rights or does she have to just leave?

The title of this thread, "Squaters rights", suggests that she may be asserting some sort of adverse possession claim to the property. If she believed that the property belonged to her late husband at the time of his death, and depending on the SOL period in wichever state she lives, that might be a possible defense.

On the other hand, it appears that she knew it was their property all along. In that case, she can be evicted (but only by the owners of the property; who don't appear to be the sister-in-law) as LisaB4657 and ebaines have said.