What do you think of me letting my twin brother move in without my housemate's permission
That probably won't go over so well. Wouldn't it be better if he stayed at your parents?
Actually my parents prefer him to live with me than anywhere else
You should always ask a housemate if it's okay to let someone else move in. When you room together it's important to follow the basic etiquette rules.
I can ask my housemate but she's probably going to complain about it. My brother doesn't care and will probably move in regardless.
I would caution you against doing this. You're housemate is going to be extremely upset if they don't agree to him moving in beforehand. If you're housemate says no your brother should find somewhere else to go.