Niece’s Fiancé’s Jewish Status

I have a situation to explain. My niece is getting married soon. She is reformed and so is her fiancé. Her fiancé supposedly is Jewish and was born and raised and is practicing as a reformed Jew. They are having a fully Jewish ceremony with a reformed rabbi and a chuppah. But I have no way of knowing for sure one way or another if my niece’s fiancé is really Jewish by Halacha. There is so much intermarriage and invalid conversion among reformed Jews that many and possibly most of them are not Jews according to Halacha. There are even some reformed rabbis who are not truly Jewish! I have inquired to know if my niece’s fiancé is really Jewish and I cannot get any conclusive answers. They are very assimilated and are clueless themselves and are offended with me asking these types of questions. I know one is not supposed to attend an intermarriage, and there have been many in mine and my husband’s families, but if I do not go to this wedding, my sister will be furious at me and might never talk to me again. I already have a delicate relationship with her and I am doing my best to keep it on good terms. My husband and I have actually been at some weddings in our families, only to find out later the people our relatives married identified as Jewish but were not Halachic Jews. This has been a difficult situation in our family because we are both baalei teshuva and most of our relatives have cultural Jewish identities but know little about religion. Is the prohibition in Jewish law against attending an intermarriage wedding a minor or a major one? Is it ever permitted to attend one in certain situations, such as to preserve healthy relations with relatives? And what do you do when there is not a certain answer, but just doubt over whether one of the partners is Jewish?