My question involves real estate located in the State of: Oklahoma

My sister and her husband Joe, Canadian citizens, are sharing (with his American brother) in the proceeds from the sale of Joe's deceased mother's house. Joe's brother has sent several documents to be completed, including a W9, which I thought was only for Americans. How do we find out what forms have to be completed and where we get them?

They have also inherited "mineral rights". Will they pay American or Canadian income tax, or both?

Is there a Canadian site like this one, anyone know?