I'm not sure of your religious background or assumptions, so I don't know what you mean by "okay." In Orthodox Judaism, as far as I understand it, men and women aren't supposed to have any physical contact that could be construed as sexual unless they are married. For some, that extends even to handshaking, but some allow that, since in our times it is so clearly nonsexual. In terms of sleeping on a bed, other than some permissibility for relatives who are so close that we assume its nonsexual (so: a father and his toddler daughter, especially if they are both clothed), we don't want any men and women sleeping on the same bed, even clothed, and especially if they're an unmarried couple.
As for the presence of the brother: while there is some room to allow certain activities when there is a 3rd party present-- so, for example, a newly married couple who have not yet consummated their marriage and cannot consummate it because the wife is menstruating, are not allowed to be alone together, but the presence of a 3rd party serves as a reminder/hindrance to their losing sight of those laws. Similarly an unmarried man and woman are not allowed to be alone together anywhere that they might engage in improper (read: any sexual) conduct. Here, too, the presence of the brother might suffice (although if the brother is no hindrance to them acting inappropriately, it is not clear why that helps). But in terms of sleeping in the same bed, I know of nothing that would allow that in the presence of the brother.
An argument could be made that if the bed were so large they would never touch, that perhaps that could be allowed, but that would usually mean that it's two mattresses, and they could be separated from each other.
In sum, then, as far as I know, there is no leniency to allow an unmarried man and woman to share a bed (particularly a couple, who are more likely to act inappropriately), and the presence of a 3rd party is what helps them avoid a completely separate prohibition, that of being alone together where compromising activity might occur.
In the interest of brevity, I must say that the Halacha frowns upon young people sleeping together before they get married. That being said, historically, young people will always act like young people regardless of what the codes of Jewish Law or books of Etiquette have to say.
Still, young people tend to forget how easily it is to get pregnant. That’s one of life’s simple complications and that is why marriage is such a great institution for young people who love one another to seriously consider. Mind you, I am not condoning the behavior; I am merely speaking about its reality in our times.
However, when you add an eleven year old brother to the mix, I think you are modeling some very poor behavior. Parents must have privacy; in fact, everyone ought to have privacy. Granted, that is not always possible. Here is my recommendation: (1) Have your eleven year-old brother sleep in the living room. (2) Do not engage in any intimacy while he is in the house. (3) Better still, have the boy stay with his parents—where he belongs.
As I was going through the classical Jewish legal texts dealing with Jewish etiquette, I came across several interesting historical facts that most rabbis and lay people are probably unaware of. In the impoverished communities of Europe, Jewish and non-Jewish families did not always have the luxury of having separate bedrooms like we now have today.
It was not uncommon for entire families share one bed —provided of course, people were modestly attired. Given the lack of heat, the circumstances were decidedly different because a sick child might not survive without the body heat of the other family members. Such a practice was very common throughout the medieval period as well. By the 18th century, the custom of family beds became widely unusual throughout most of Europe. In the United States, the practice continued well into the early 20th century. Even Abe Lincoln, while traveling, would share a bed with a friend.
My, the world has changed . . .
Given the problems we know recognize about pedophilia, we can now say in retrospect the medieval practice of families sharing a bed or a bedroom ought to be strongly discouraged.
 Even HaEzer 21:7.
 Jeffrey L. Singman, "Daily Life in Medieval Europe" (Santa Barbara, CA: Greenwood Publishing, 1999), 46.
 Stephen Mennell, “The American Civilizing Process”(Oxford: Polity Press, 2007), 64-65.
The very fact that you are even submitting this question means that at some level you already know the answer. No, it is wrong, on multiple levels.
First, an unmarried couple sleeping together in a room in which there is also an eleven year old boy is wrong. Even if you do not engage in any sexual behavior, this is simply inappropriate. At best, it sets a terrible example for this child. What positive lessons do you think you will be teaching him?
If you also are thinking that you would engage in intercourse, or even sexual behavior or any kind (including kissing), in front of him, it becomes not just inappropriate, but unacceptable in every way.
The fact that this is the 22 year old woman’s 11 year old brother just makes this entirely wrong – nor would it be better if it were the 22 year old man’s brother. This child is old enough to see what is going on, and may be very negatively affected long term by being exposed to sexual acts.
As for the issue of the 22 year old unmarried couple sleeping together in the same bed, the answers also seem clear. You are both beyond the age of consent, so there is no legal barrier in civil law. Jewish law would frown on it on grounds of tzniut (modesty).
In regard to engaging in sexual behavior with each other, under the rubric of sex being something that is to be considered holy/sacred, and to retain that status should be confined to the bounds of marriage (or between committed partners), that is a decision that each of you and both of you together will have to make.
Give your question, however, it seems to me that you are thinking that having the younger brother in the room would lend cover and permit you to engage in behaviors that you would not otherwise undertake for fear of discovery. If that is the case, you already know that it is not something you should be doing.
It is possible that I did not understand your question correctly: if that is the case, please resubmit it with clarification, and I will try again.
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