You tal about lots of family problems, but what has really happened?
1) Your brother stays out late & your parents are upset. You havve no idea of the relationship of your brother & this woman. And there is nothing you can do to change it. He is 27 and going to do what he wants. If he were not getting something from this relationship, he would not be doing this.
2) You say you are getting married, but are scared. If things are that bad to scare you, why are you getting married ? This is the only problem you can correct. It seems that you need to talk to your future husband. This is a stressful time for everyone - these people are going thru huge changes - their son will marry & bring a new person into the house who doesn't like anything in the home. This makes a lot of tension, & will cause stress. The future in laws are the people who made your future husband the man his is, & the ones who raised him to be the man you love and who will be the father of your children.
Relax & try to get to know you future family. Invite your future mother in law to lunch, go shopping together, & do whatever you can to fit into the family. If not, the marriage will be difficult.
wishing you the best,
lady moon