I think this is the idea time to be truthful. Unless he is totally happy not having ANY toys that are age appropriate for him and will only play with hers.
Her throat is about the size of a drinking straw. That is why her food has to be cut so much smaller than the bigger kids. Balloons should never be allowed with kids under 5. If they get a broken piece in their mouth and it chokes them the paramedics can't get it out. When it gets wet it gets slick and they can't grasp it to pull on it. They die from choking.
She is at risk of choking if he has any toys that will fit inside a toilet paper tube. That includes Hot Wheels cars, small bouncing balls, jacks, lego's, any toy with a magnet in it that could pop out, toys that make sounds have tiny pieces in them that can come out just by being banged on the floor, any toy with wheels that could break off, squeekers inside toys, eyes, noses, buttons, beads, etc...on stuffed animals, toys that have cords that could be swallowed or accidentally wrapped around the neck, no toys with batteries since they can come out easily and be swallowed, crayons can be chewed up and swallowed and choked on, board games, marbles, any toy that can be broken or taken apart.
This is just a partial list of the things he will not be allowed to have that are age appropriate for him. She will get in them even if they are up in the top of the closet. We had our puzzles and games up in the girls closet and they fell one time when I was in the shower. It woke her and her little brother both up and she was smart enough to close her door and come get me. She protected him, but she was about 5 or just about 6.
Your little one should not have to do without his toys and the things he needs to play with to develop and grow. They need to not share a room.
If you cannot move to a place where they can have separate rooms then I encourage you to keep her in your bedroom until you can move to a place where they can have their own space. She is young enough he can have his toys and a baby gate be put up to keep her out all the time, even at night. She will get out of her bed and wander before too long if she is not already doing so. She needs to be safe and if his room is not completely off limits she is at risk.