Is there a way for a us citizen to claim in-laws in order for them to immigrate to the us. Can this be done or does it have to be done by birth children only. If so, can they be claimed as dependants, and be able to receive medicare?

In order to sponsor your parents you have to be a US citizen over 21 years old. I'm not sure about sponsoring your spouses parents but I would assume that the spouse (the child of the inlaws) would have to at least havea green card.

The US citizen (you I assume) would have to be able to support the in-laws until they become permanent residents. Read the link below and call USCIS to check

Immigration through a Family Member