Is it okay to be nice and normal to an ex, who has done something very very incorrect and adharmic with us in the past, even if we have not been able to forgive his actions?
There’s no point, spiritually speaking, in behaving rudely to anyone. Politeness and ordinary human kindness should be natural to you in respect to virtually everyone (who isn’t otherwise a present threat).
There’s a difference, too, between forgiving and forgetting. So long as someone is around who might still even harm you, you mustn’t fall into the same trap and be harmed again. Most people don’t forget such things and probably shouldn’t. But forgiveness which flows from a divine power, a grace, is right and good.
If by accepting your “ex” into your family associations might in time help him to be a better person, well, maybe that’s good. But be as Jesus counseled “Wise as serpents but harmless as doves.” Don’t be “merely” polite when your thoughts remain less than polite. That’s being hypocritical. Indian culture tends to wear the mask of politeness perhaps past the point of self-honesty!
In matters of principle you should be strong. As I do not know the circumstances of which you speak, I cannot say and would not therefore suggest that your “ex” be banned from your family circle. But one who has harmed others and who is sincere should think in terms of making amends. And by perhaps pretending that nothing happened you are NOT helping him to deal honestly and sincerely with his error.
I say all of this because you used the word “us” and I assume therefore the “us” refers to your family. Perhaps your family, then, should discuss this question as to what is the dharmic attitude towards him. Your consensus would make it better able to act together so that you, the ex-wife, are not acting out of anger or revenge but seeking dharma.
If you can help him, then accept him; if by accepting him you “enable” his dishonesty, then you must firmly but kindly stand by principle.
Blessings to you,