that girl next door,

You are confused about how to make your relationship with your married sister better?

This is what I gleaned from what you posted:

It seemed that your best friend got in between your sister and her older husband.  Your sister became very close to your friend.  Your sister got annoyed (at you?) because you did not tell her what her husband said about your friend.  She is also annoyed that her husband is interfering with her relationship with you?

Now your sister does not want to talk with you and you want to find a way to make things better with her.

My question is that have you tried talking with her (rather than email or text)?  You can let her know how much you miss her, how you want a close(r) relationship with her and how you want to support her.  You can just be an empathetic listener.  You need not share to her what you feel about her husband for that is no point doing that.

There is a saying about communication: you ask three questions to yourself before speaking.  Is it true?  Is it useful?  Is it kind?

Mark