Sometimes after not seeing my mother in-law for a long time, I say Salaam to her and kiss her like my mother, and she is elderly. My wife also kisses my father, is this permissible?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

You have to know, may Allaah enable you to do good, that your mother in-law is an eternal Mahram to you (non-marriageable woman), so there is no harm in you kissing her unless there is an exception, for instance if you have some unnatural feelings towards her, in this case it becomes forbidden. The same thing applies to your wife with your father. However, one should avoid kissing in the mouth in these cases, and one can kiss on the head, between the eyes, or on the hands.

Allaah Knows best.