Hello Twilight ~ welcome!
I'll recap:
You have known your girlfriend for almost 3 Years and have dated her for nearly as long.
You say that you love her more than life itself.
You say that you feel that she is the love of your life.
You say that she is sweet & beautiful, and you believe that she really cares for you.
She listens to you & understands you.
You feel that she has a general loving glow towards you.
You connect very well in conversations.
Yet, sometimes you wonder if it's really worth all the trouble.
The problem is that not all of your memories are pefect.
You have had your share of troubles in the past
You have a best friend, who seems like a sister, whom you have known for about three years ~ a little longer than you have known your girlfriend.
You say 'we're Practically Sisters'.
Your best friend, whom you think of as a sister, absolutely hates your girlfriend and wants nothing to do with her.
Things have happened in the past that have caused your 'sister' to have a deep hatred for your girlfriend
But your Girlfriend has already Forgiven best friend / 'sister' for her part in that matter.
Your say that girlfriend has faults ~ sometimes she seems to be extremely vain and overly focused on herself.
She has cheated on you twice in the past, but has owned up to it and says that she feels awful about it.
You have known your best friend, who seems like a sister to you, for about three years ~ a little longer than you have known your girlfriend.
You feel oddly attracted to your best friend ~ the one who might as well be your sister.
You have felt this way for a while, but have never really paid much attention to.
You consider it sad that, even if you told your best friend how you feel about her ~ ie as more than a sister ~ you think that it would make no difference because you believe that she is determined not to love anyone.
Consequently, you feel that saying something would make things a little awkward between you.
You want to know if you should ignore the feelings that you have ~ this 'Odd Attraction to My best friend'.
You want to know what you should do about your girlfriend.
You have felt forced in the past to choose between these two girls and consider that to be 'awful'.
Do you really have to choose one or the other for things to be peaceful like they used to be?
How can you help them to get on better, so that you won't have to be put into the difficult position of having to choose again?
*
I've tried to set out your points in an order that I feel I can respond to, and here are some thoughts, comments, questions:
I think that people do love their best friends and sisters, but not in the same way that they love their girlfriends, so I think you need to give some consideration to what your feelings actually are.
I know people who are 'like sisters' and it's usually because they have been brought up together ~yet you have known this girl for only three years.
You even call her your sister ~ yet you also say that you are attracted to her. Perhaps by saying that she is your sister / best friend, you have been able to have two close relationships with girls you find attractive, without anyone being able to accuse you of two-timing ~ had you thought of that?
I also think that it is possible to be attracted to more than one person at a time ~ the important thing is how you deal with it. Some cut the one person out of their lives completely, some have affairs, some stay 'just good friends'.
Your girlfriend has cheated in the past ~ I'm wondering if you think it might be ok for you to cheat in return.
You say that you love your girlfriend more than life itself & that she is the love of your life ~ yet sometimes you wonder if it's really worth all the trouble. This a bit bewildering. It seems that you have come to terms with the cheating but can't cope with her being 'vain'.
What do you mean by this vanity, I wonder?
Or have you not got over that cheating? Have you not yet forgiven her?
You also say that it's sad that if you told your best friend how you feel it would make no difference because she is determined not to love anyone.
Why is this sad?
Because she will never find love?
Or because you can't have her?
But if you have a girlfriend, you can't go after someobne else too, surely?
Is it possible that your best friend is actually attracted to you and that is why she is not seeking love elsewhere and also why she hates your girlfriend?
You need to decide which of these girls you really care for. I don't think that it is going to be easy for you to have both of them so closely involved in your life ~ not unless both are very easy-going people ~ and it seems that they are not.