Let him sleep with you or with his parents.
There is no need to force his brother to be a parent to him. That is not fair and will destroy their relationship.
If you are worried about a five year old sleeping in the same room with adults, read the book The Family Bed, and also, about brothers fighting in general read, Raising Our Children, Raising Ourselves. Amazon has these books.
Have your daughter read them too so you can agree on what you do.
Have you tried splitting up the bedtime? Maybe put the 5 year old to bed first and then an hour later put the 7 year old to bed? May take a bit of time for the 5 year old to settle on his own but it will save the 7 year old from getting all upset if he can sneak in when the 5 year old is asleep?
Of course no end of yelling and screaming from their parents won't work - yelling and screaming at a five year old is not the way to go. Separate their bedtimes and make sure the five year old always has the same bedtime, routine is essntial to security. The problem here is that the five year old is insecure because of all the past problems and shouting and punishing will only make that worse. Make sure there is a set bedtime routine - supper, bathtime with lots of bubbles and fun and games, into bed and a storytime. settle him down properly with a story, read him to sleep if necessary, give him his own personal secure bedtime and put the seven year old to bed an hour or so later once his brother is asleep. Family councelling may really be necessary, not just because of the bedtime thing but because there is obviously a lack of parenting skill and insecurities to be resolved.
