I don't want to befriend my brother's much younger girlfriend because it makes me uncomfortable
Why does it make you uncomfortable?
She's a bit too touchy feely with me.
It is understandable to be uncomfortable around young girls acting a sort of way with you. You are doing the right thing distancing yourself from her.
Agh! It doesn't matter at all so I don't wanna talk about it.
You're fine if you don't want to befriend your brother girlfriend. You don't have any obligation to do so.