Why should you feel guilty? She had her chance - she chose not to have him. You've been honest with her and she was the one who told you it's ok. So go for it. Cus ultimately, if she's a sister to you at all, she should eventually realise you two are happy together and if she truly cared for either one of you, she'd want you two to be happy. Unless her reasons for not wanting you with him is that she knows he's not actually a nice guy or you two aren't right for each other, I say date the guy. You shouldn't feel guilty. Your sister's feelings are for her to sort out.