throwaway account for safety.

So, my girlfriend and i are, i guess you could say, slightly starting to plan our wedding. we know for certain that we want to be married. but we're waiting until our lives are more settled to make it official (we both have new jobs and are currently house hunting, other stuff, etc.) and one of the topics that came up is who we would want as the bridesmaids and groomsmen, and she said maybe i should consider her brother. 

i flat out refused because he's terrible. he was spoiled and sheltered his entire life and is just a weirdo and wildly inappropriate in any social setting. at every single family event ive been to with him there, hes embarrassed himself and his family. at a fancy dinner once for my girlfriends graduation, he purposely spilled pasta sauce all over his shirt and laughed about it. at a quiet family get together, he took off his shirt and started rubbing his nipples and moaning, at another family event, he told his ten year old cousin what porn is and how to find it and then defended himself by saying he was joking. i culd go on for hours with these stories. as far as i know, he doesnt have any mental disabilities or anything like that, i work with developmentally disabled people as my career and this guy is just a piece of shit who wants attention.

 i told my girlfriend that i dont want that because hes embarrassing and i have close friends and family that i would want as my groomsmen instead. she was alright with that. but then i started thinking about him at my wedding and i realized that i really honestly just dont want him there. i talked to my best friend about this and he said it probably would be rude to just flat out not let him come, but if he started to act out, to kick him out. and that seemed like a fair compromise. i told this to my girlfriend and she had a shocked look on her face but when i was honest and explained myself, she (probably reluctantly) agreed. shes very non-confrontational though so im not sure if shes really okay with it. another thing i worry about is their mom. she treats the brother like hes a perfect angel and is the definition of entitled parent. so she might cause a scene if i throw him out. and i wont kick him out for making a bad joke or something. i mean something really bad like the nipple thing or something on that level. so would i be the asshole if i did that? thank you for your opinions