ANSWER:
You must not meet her nor socialize with your uncle’s wife, whether she be a muslim or not as she’s a non mahram to you.
Your uncle is committing zina if he acknowledges that their marriage is invalid. If he believes that it is valid to marry a Hindu, he is an apostate as he is rejecting the Quran and Sunnah! Your parents must be warned that they may share his same fate if they acknowledge that this is a valid marriage.