My Girlfriend’s brother beats her up, i got involved, what shall i do?

im 18, my gf is 18 and her brother is 20.
Their parents have gone away for the weekend, and they know ill be sleeping there, and having dinner etc.
but this morning my girlfriends brother came downstairs, and started saying that i was eating all their food, and i was ponsing off them,
then my girlfriend said “No, mum said luke can stay as long as he wants while were gone”.
so me and my gf went to her room, and he started shouting from downstairs, my gf went down and they were arguing, then i heard her crying and screaming in pain, so i ran downstairs, and he had her in a headlock and was punching her, i pushed him off and he wass gunna throw a punch at me, but my gf slapped him hard in the face.
we managed to go back to her room, but he came up the stairs and through one of his boots at me, and then chucked my gf down the stairs, he came to her room, and pushed me on the bed, i kicked him in his stomach.
he went to my gf(laying at the bottom of the stairs) and they fought a bit more( i tired my best to get them away from eachother).
i left and i said to her “dont worry, ill get my dad up here”.
my dad drove up in his car(to their house) and had a verbal argument and shouting with my gfs brother.
i said to her, “why dont you just come down to my house, have a wash and get dressed there?”
she said ” no, ill have a shower here”
i was like :S?

so my dad and myself went back to our house.

i dont know what to do, ive called her ( she aint picking up) ive text her asking what is she doing?

any help please?

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