I have a horrible relationship with my sister and non-existent for many years, so I guess be happy that you had a close relationship with your brother back then. I really think with risk of sounding bias, that women are destroyers of a lot when they decide to settle down with a guy. That guy's past life seems to evaporate once their new relationship starts, or at least it does after some time. With that new woman, if it seems if she is not your normal type or it seems strange somehow, I am guessing you won't be with her long-term, unfortunately. I imagine if you are nuts about someone, you would know it pretty instantly. At the same time, I could be wrong, if you like her, I don't see any need to break it off. I am older now and kind of washed up, and no friends or women in my life for a very long time. I slept with a few women in a short period of time at one point and i know what you mean, it is not satisfying. Still to be with 5 women in that short a period of time to me is pretty amazing. At least you had that intimacy with them, as short-lived as it ended up being.
I have so little faith in people and relationships lasting that I wouldn't be surprised if your brother and his girlfriend end up fighting and him moving out eventually. For all we know, him and his girlfriend are temporary and before you know it you could hang with him again.
I wouldn't even worry about trying to get married, or at least not think of that as the end goal. If that many women were willing to be with you in that short a period of time, you are miles ahead of me and probably a lot of other people on this site. That girl you are seeing, I guess for now hold onto her for dear life, even if she isn't maybe perfect. I am so desperate that it is really whatever I can get now. Looks or personality are not anything I could be picky about right now
Anyway sorry about your rabbit, without my cat, i don't know what i would do, i often feel like i can relate better to her than any human being