My girlfriend is a gold-digger
My girlfriend and I are not on good terms. She shows me love when I give her money, but if I don't give her money I don't generally hear from her. Her sister has a crush on me and what I expect from my girlfriend, her sister is giving it to me mostly. I am confused.
I think I understand what you are trying to say. Your girlfriend loves money. Every time you give her money, everything is sweet and dandy, but when you don't give her money, she becomes bitter and she would not even call you.
Her sister, on the other hand, has her eyes on you. She has been showing interest in you. That might have been going on from a long time and she might be saying in her heart, "My sister is a fool. Why is she treating this good man so bad? I am going to take him away".
You have seen how interested she is in you. It is evident that both of you have got together. I think it is despicable for you to yield to her temptations.
You should not have become intimate with your girlfriend's sister, regardless of how many times she might have thrown herself to you.
Keep away. Say no to her at all times. She may call you a 'fool', but it is better for her to call you unflattering names than for you to go to bed with her.