I think you should consider both parents feelings. Obviously your mother whether she knows or does not know about the affair.  And your fathers feelings as he may have made a mistake or had some other reasons that would be hard to explain.

The best thing to do is not take it personally. Obviously your love for your mother will be overwhelming. And the feeling of betrayal must be constantly on your mind since you found out.  And some men really do love their families and do things which they shouldn't.   However, support your mother in whatever decision she makes and stand by her side. Since, if as you say, your society is conservative then she will really need your love and support.