I want to give away my sisters cat.
Why would you give your sister's cat away?
I think our brother is allergic.
If you think your brother is allergic you should see if you can confirm before making a decision. You should never give peoples pets away she might be able to find an option that works for everyone in the situation.
Whatever, I've clearly decided.
Please remember it isn't your cat. You don't really have a say in this. Also Benadryl is pretty cheap and could be a workable solution for him.