Don't discuss your future husband with her and ignore her if she brings the subject up. Meet her abuse with silence. This the best you can do quiet things down.
Her talk of your sister having to marry first is nonsense. She is using that as an excuse to keep you from leaving home - and her. She has lost a husband and now sees herself as losing a daughter. Unless she has a history of being abusive, she will calm down in time. Just don't add fuel to her fire. And go ahead with your wedding plans.
This is a choice you're going to have to make. Is your mother more important to you, or is your boyfriend? You could be 85 years old and single waiting for your older sister to get married.
I can't understand the concept that siblings have to marry in order. That puts tremendous pressure on your sister. How does she feel about this?
You have to have a talk with her and tell her that her behavior is not good and you are going to marry your bf ...put your foot down ,you are 25 years old its time to make a stand. If she still continues I suggest you move out you will be doing so when you marry anyway, so be firm do not take any abuse .by the way Congratulations its good to hear couples are still getting married .