Hello everyone

So here is my little situation. I have 6 bridesmaids and 2 groomsmen haha. My groomsmen are my fiance's brother and my cousin-in-law who I care for a lot. I've been having a little debate with my mum because she says I should have my best friend's husband as a groomsman, she is one of my bridesmaids and just recently got married, but I have yet to meet him and have only seen him in pictures. I keep telling my mom no that I wont do it because I dont see why I HAVE to, she says its so that they dont feel left out, but isnt the bridal party supposed to be people you know and see as family (if they arent already?)? My fiance sides with me on this one because he hasnt met the guy either and he feels that he would feel awkward having a complete stranger next to him at the wedding. We dont mind him coming to the wedding at all, Im actually glad about it, but I just see him coming as a guest, not as part of the party eventhough his wife is....
Please let me know what you think, should I side with my mother and ask my best friend to let her hubby know he's part of the wedding or should I stand my ground (in other words, am I right with saying no?)
Here's another little problem I'm having (sorry). My sister had been seperated from her husband and she recently got back together with him, I (and most of my family) along with my fiance cant stand the man and dont want him in our lives, but Im afraid that if I tell my sister that I dont want him at my wedding that she'll say she wont come. What should I do? He's never cared for me or for anyone in my family and whenever we have family get togethers he complains and always wants to leave early. Its my day, I dont want it to be ruined by someone like that. What should I do?
Thank you for listening!