Dear brother,
Congratulations on coming into the church! I’m sure you’re having a great Easter.
There are some problems with your plan. The main one, as Richard pointed out, is the problem of scandal. People will assume that you’re sleeping with your friend, and that it’s okay to do that because, hey, you’re a Catholic so it must be cool. Even though you’re not, you don’t want to be explaining it all the time to people, especially since they may not believe you.
The other problem is that your friends are all atheists, and it would be easy to let their beliefs rub off on you. Hopefully it would be the other way around, but there’s a risk. Satan is hanging around, trying to get us to lose our confidence.
I would hope you could find some people your age in the church to share a house with.
I have to comment on something you said: “I told him to marry her but as an athiest he really has no reason to.” So when I see a guy living with a girl he’s not married to, my first impression is that he doesn’t really care about her, he’s just using her until somebody prettier or more interesting comes along. He doesn’t respect her enough to marry her.
Marriage is for the protection of women and children, so that women don’t end up getting dumped later in life, and children don’t form attachments to a series of men who come in and out of their mother’s lives, which can really mess them up.
Your brother and his GF have no children, sounds like. Good bet they’re using birth control. So he’s saying to her, I like you enough to sleep with you but not to have kids with you. That, after awhile, can damage a relationship; Catholics believe birth control is responsible for much of the divorce in the world, because it puts such a barrier between spouses.
I hope you can find a better place to live, and that your brother will see your good example of a holy life and be inspired to copy you.
I think you can still receive the Eucharist, though it might be best to check with your priest. Have you tried praying for another place to live? God may have a plan for you – He is interested in your life, you know.
God bless.