Hi, I get the impression that you are sensitive young man and that your family means a lot to you. I can empathise with this as I am very close to my family too.
The thing is, if ever there are problems with members of your family, you feel that you have to try to solve them. Sometimes, of course, you can ~ but often, you cannot.
I think, from your post, that you sister has left home, with her boyfriend of 4 months, whom she wishes to marry.
Your father has gone after them & your mother is very upset.
Other than that, I don't know the situation.
Four months is not a long time ~ and I would recommend that any 'child' of mine waited longer before deciding to marry, but plenty of people
do feel sure that they have found the right person in that short amount of time; and many are correct.
A lot depends on the age of your sister ~ and the age of the boy ~ and their personalities.
Are they old enough & mature enough to make this decision?
Is he a nice person, who can be trusted to care for her?
Are they safe?
Why do they feel that they have to run off?
Are your parents overly strict or difficult to talk to, perhaps?
Do all involved need to try to see things from the other's point of view?
Ultimately, you cannot control how your sister or your parents behave, but you can talk to them & help them through difficult situations, and to understand different viewpoints ~ and you have your girlfriend who is a support to you as well.
You say 'i can't do it this all my own' ~~ what is it that you feel that you have to 'do'?
Good luck!