What is the ruling on refraining from attending an iftar invitation in Ramadan extended by the wife's father (the maternal aunt's husband) due to family problems and their undesirable characteristics?

Attending the aforementioned invitation is not obligatory according to the more preponderant opinion among scholars, and attendance is only obligatory if it is a wedding feast. However, if your presence holds a benefit, such as advising those present and enjoining good upon them and forbidding evil, then this is a good deed for which you will be rewarded. But it is forbidden for you to sever ties with your relatives, even if they treat you badly, for in the Hadith: "If you are as you say, then it is as if you are feeding them hot ashes, and there will always be a supporter from Allah with you against them as long as you remain in that state." The meaning of this is that the pain they suffer due to their actions is like someone eating hot ashes, and there is no sin upon you, rather the sin is upon them.