<human>: Condense the text into a few key points

My mom has been dating this man for around 2.5 years. He is great. So nice, so generous, funny, interesting, adventurous, kind etc. You can't help but liking him...honestly, his only flaw is that he is with my mom. My aunts, grandma and everyone else can't figure out why he is with her. She has major mood issues and a problem with alcohol. 

Her bf lives with us on the weekends and during those times, my mom accosts me with uncomfortable questions in front of him and others. Usually while drunk, she'll AGGRESSIVELY confront me about "why I hate Dave"- (I don't, I really genuinely like him and continue to build a relationship with him through mutual interest- he is an architect and I am studying to be one and we both play a lot of instruments and love our dogs) 
these questions make me really uncomfortable and I never know how to respond so I just leave the room
recently it's been getting worse- like he'll come into the kitchen and we'll say good morning and she'll walk in a minute later and the first thing she'll say to me is "have you said good morning to Dave?"

she also does this to my little sister (16), but in an even more aggressive way- mainly because my sister responds like any normal teenager would (with hostility) and my sister will start crying or run out of restaurants etc

I don't know how to address it- she is also really hard to talk to because of her mood disorder- manic depressive bipolar (untreated) and her highly evolved alcoholism- she gets EXTREMELY combative if you try to talk to her about her behavior and is irrational- I also want to help with my little sister but don't know how- do I talk to dave? 

thanks so much for all the help!
<bot>: My mom makes me and my sister feel uncomfortable regarding questions about her boyfriend...how do I act to mediate the situation?