It will get messy. My brother went out with my best mate and then finished her, I no longer speak to my mate as when she started going out with my bro she stopped coming to see me even though she was at my brothers every weekend. Then as soon as he finished with her, she wanted to stay at mine and use me to get to my brother. When it came to my birthday party she promised That she wouldn�t cause any hassle with my brother and she came out and made a big deal about things and walked out, I didn�t let it ruin my night, then the next day she sent me an e-mail asking how things were I said I had a great night then she said �Oh don�t bother asking how I am will you� I havent spoken to her since because I wouldn�t take her side over my brothers and have her using me to get at him, plus she was a selfish b1tch. So to me going out with your sisters mate is a big no no. Its not fair on your sister as it will cause arguments. And for you to still be mentioning your ex in a question that didn�t need it you�re still not over it. As has been said before work on yourself but if you really feel the need to have a girlfriend go for someone new that isn�t part of you past or present that can be a part of you future.