Answered by Dr. Marie Hartwell-Walker on - Link
Although you said that there is no reason for your feelings, I disagree. Your experience at a young age is that love puts you at risk of loss. Your parents couldn’t sustain their love for each other. You were abandoned by someone you loved. Your mother was so stretched that she couldn’t give you the love you needed. It makes sense to me that your inner self decided that it’s better not to take the risk of loving someone because it only leads to abandonment and pain.
I suspect that your problem with your sister is the opposite of what you think. It’s not that you don’t love her. It’s that you actually love her very much but that terrifies you. Deep inside you believe that she will abandon you too, or at least not be able to love you back as you need her to. So you shut down your feelings and “abandon” her emotionally before she “abandons” you. You find reasons to envy her as a way to be emotionally distant.