Thousands if not millions of children share bedrooms every day. Its not only legal, its common. Not all parents can afford a place where everyone has their own bedroom to themselves.

But one being only 18 weeks old... its the older poor son that's not going to get any sleep. Why don't you keep the baby in your room until he is able to sleep through the night.

It is not illigal for any siblings of any age to share a room. And for boys its very common to share a room all the way to adult hood (though girls and boys can share a room, its better to separate them at puberty to give each child the privacy they need)

I agree with smoothy though, your 11 year old is not going to sleep well with the 18 week child in the room. I would move the infant into your room until he is sleeping through the night