Is refraining from sharing in my aunt's joys and sorrows considered a severance of kinship ties, given that I know she secretly married off her daughter to her aunt's husband, despite her knowledge of the prohibition of combining a woman with her paternal aunt, and this marriage is still ongoing?

If a man marries his wife's paternal aunt or his mother-in-law's paternal aunt, this marriage is void and they must separate immediately, based on the Prophet's (peace and blessings be upon him) saying: "A woman and her paternal aunt, or a woman and her maternal aunt, cannot be combined [in marriage]." This matter is unanimously agreed upon as forbidden. As for the children, they are attributed to him because it is a case of intercourse due to شبهة (misapprehension of legality).

Regarding visiting the maternal aunt, if she refrains from this forbidden marriage by boycotting it, there is no harm in that. However, if she does not refrain from it, then maintaining ties with her while advising her and reminding her of Allah is more preferable. It is incumbent upon whoever is able to denounce this evil.