You do not know all the facts yet, so do not make assumptions. If he has been unfaithful what will you do? I think you are old enough to cope with this in a mature way. It is also best not to put yourself in the middle, it ill only hurt you more in the long run. Try to support your Mother and be there for her, as that is all you can do.

My father cheated on my Mother all throughout their marriage and even had an illegitimate child with another woman. He quickly got rid of her after the child was born and found another woman to sleep with while my Mother tried to please him in every way. In the end she finally kicked him out of the house and he killed himself some months later.