Sister's secret engagement

I know this isn't exactly wedding planning related, but I love all the feedback this community gives, so feeling like it's not the most inappropriate place to post.

Alright, so before getting into a summarized backstory, FH and I got engaged in December and my family has all been very excited and involved in planning our wedding, as well as paying for most of it. Last night my younger sister (of 2.5 years) told me that she has been secretly engaged to her undesirable boyfriend for a year and I don't know how to handle it.

My sister has had some medical issues in the past, and while she's in her early 20's she is developmentally more like a teenager right now and not super sure if she will continue to mature or plateau. She has been dating her boyfriend (fiance??) for about three years but maybe 7 months into their relationship my dad banned him from the house because he's crazy (thinks humans need to be exterminated, weird alien things, very weird political views, DOESN'T SHOWER, among other things). About a year and a half ago he became homeless (because he got fired from his job for being late/missing because he was playing video games), and my family is major drama about her always hanging out at the shelter, and with his friends who don't have jobs either(?) and just play video games all day.

Probably too much info, so anyway I don't really know if I'm asking for advice or what, but most of the people in my life are pretty close to the situation, so already have quite biased opinions about the whole thing. I just don't even know how to respond because I don't support their relationship, but I want to support her as a sister.

Wishing we were planning weddings together instead of this super secret bomb being dropped on me I guess. Thanks for listening!