Look, I don't have time to be your QA team. I am playing a campaign and I tend to overwrite saves as I go along. Here I will make this easy for you. Translate this text and send to QA if needed.
1. fast forward until younger brother becomes of age 18.
2. Try to marry older brother.
You will only be able to marry the younger brother. This is so simple it makes a 6 year old blush.