so my siater-in-law can only get s2 visa. for s2 type, medical is needed or not?

Khan, Chinese embassy in your country doesn't mention medical check for S2. So I think it isn't required. If her S2 expires, then she can try to ask for an extenstion at local entry and exit management bureau. If they don't extend her S2, then she has to get back to your country to reapply.

Please understand that 180 days is the longest duration of stay for S2. The consular officers can decide how long she will be allowed to stay. That is to say, she may get shorter duration of stay.