I agree with @Simone_De_Beauvoir, so I don’t need to say that again. Let me talk about some of the implications of the social construct. Since it is culturally determined, the consensus about sanity changes from subculture to subculture, from community to community and from nation to nation. Unacceptable behavior in one place may be considered completely sane in another.
Sanity also becomes a synonym for normal. We are normal. We are sane. Those others are insane. They are so different, they can not be responding appropriately to situations. Thus do Muslims become insane (they are all suicide bombers, right?) Or Northern Californians or any other culture that seems to participate in strange or barbaric rituals.
Of course, sanity is different from normality. Someone could be at one extreme end of normal and be perfectly sane. At the other extreme end, they would not be sane. What I mean is that one can be abnormal and sane. But one cannot be normal and insane.
Society is willing to accept abnormalities of some kinds (genius for example) and include those people among the sane because they behave normally in other areas. Other abnormalities are indications of insanity—the ones that seem to harm the individual or society. By extension, sanity is where a person does not harm themselves or others.