Why does it hurt my feelings that my two sisters are so close?

My oldest and youngest sisters are like best friends. I never really had an opportunity to be close to either because of life situations (moving in with husband while they stayed home etc.), although we maintained good relationships. It seems now that I want to be closer, they have no interest. I'll admit I'm not perfect, and have been through some hardships that's made a big impact on me, but I've never done anything spiteful, rude or malicious. Now it seems they're closer than ever, while they're completely ignoring me. I'm happy with my life with my new husband, and can go on without their friendship if needed, but it would be so nice to have it. I'm trying to be happy for them and let it go, but everytime I read or hear something about how close they are now, it brings back those hurt feelings. It seems that unless I'm perfect and don't screw up, they're allowed to be mad at me or something. Neither ever call me, and recently one has even de-friended me on facebook, while they other let me go on the phone dismissively and said she'd call me back and never did, that was a month ago. I don't know how to approach this, as I feel like I can't make a mistake with them. I thought sisters were supposed to be close. Anyone else understand what how I'm feeling? I'm thinking I should move on and just not care, and save myself the heartache of it all. I love them still, but I think they've made it very clear they don't care about my feelings.