Ok, this whole situation is confusing me. I'm a Christian, and I've always been raised to wait until marriage. I have a twin sister and she is waiting until marriage; I, however, haven't decided if I'm waiting for love or for marriage. However, even though my sister has told me her WISHES that I would wait until marriage, she would never force those opinons on me, and she would never never judge my boyfriend because
I decide to have sex with him. My boyfriend doesn't think for me, I do. And the same goes for your girlfriend--she made her own decisions, and just because she "comes clean" about them, doesn't mean that you're the bad guy.
Your ex made the decision to have sex with you. That was her decision, so to tell you that you need to get her sister's blessing since you guys had sex... well I'm sorry but that's just plain stupid. Maybe her head's been so polluted by all the "rules" of Christianity, that she's blind to what's really going on around her.
Seems to me that her and her sister are too focused on acts, not on God. The acts come as a result of being focused on God---GOD doesn't come to you because of your acts. I don't know if I am explaining this right.. but I'm trying.
Basically, it is not in your girlfriend's place to have her sister judge you.
If you want to talk to the sister, to try to get your girlfriend back, don't try to win her support. Tell her this is absolutely ridiculous, and her sister should be able to decide whether she CARES about you. I mean, you can quote scripture if you think it would help:
Matthew 7:1:
For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you. Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye? How can you say to your brother, 'Let me take the speck out of your eye,' when all the time there is a plank in your own eye? You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother's eye.
I hate when some Christians feel they are superior to everyone. No one can judge another person because if they're a Christian than they believe that everyone is born into sin. And EVERYONE knows that not a single person out there is perfect.
I say tell your girlfriend that if she cares about you, then you guys should be together. If she needs her sister's approval, then I think she has a lot of growing to do, and maybe you should just give her some space, because this is only going to hurt you.
Basically, I'm rather annoyed with the situation, and I feel bad that they're putting you through it. The older sister had absolutely no place to condemn you.
Ok, I tried to give the Christian perspective on this
But geeeez, this situation is absurd
AND, as an answer to your actual question, because I know thats what you've been looking for..
I know my sister would give you her approval because my sister wants me to be with someone who treats me well.
And I feel the same way towards her--I just want her to be with someone who loves her and demonstrates that love.