My son is 5.5 months old. My sister-in-law always kisses him on his lips. I do not forbid her as I fear she will do it more frequently if I forbid her. So is it permissible to kiss on lip as I myself do not kiss on lip inspite of being mother because I think it one's wife right?

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.

There is no harm to kiss any part of the face of an infant even his lips if this is done as a sign of affection and mercy. Generally, the relatives of an infant kiss the infant by affection and mercy. Please refer to Fatwa 90009 on this regard. However, it is not permissible to kiss the child for desire, if there is any doubt on this, then a person who is doing that should be prevented. Nonetheless, one should be wise when admonishing and he/she should choose good words in order to avoid any unpleasant consequences. 

Allaah Knows best.