lokithor 
I am so very sorry that you grew up in such an unsafe and violating environment.
I am now highly suspicious of the "sex positive" movement, basically for all the same reasons you describe.
I am very liberal around sex and sexuality when it comes to adults, but I believe very strongly that children should be protected from adult sexual behaviour.
I cannot say too much for reasons of confidentiality but the situation you describe is sadly familiar to me from other people whose lives have been harmed by inappropriate exposure to adult sexual activity.
I also had an experience of sleeping overnight in a relatives bedroom when i was maybe 8 years old, along with my siblings, during a party where the adults all got very drunk. At some point a drunken couple crashed into our bedroom and started having sex against the wall. I found it deeply distressing and it only happened once and was not a swingers party or anything like that.
I can only imagine how alone and frightened you must have felt if this kind of thing happened in your home. To be abused by your brother as well, that is so awful.
Obviously your brother will have been damaged by this also, not that it excuses his behaviour, but I can imagine that the whole experience must have left you burdened with all kinds of complicated feelings and feeling very alone and unsupported as a child.
I am personally not concerned with what consenting adults get up to in the bedroom, but whatever kind of sex lives adults enjoy should be kept well away from children.
Any counsellor or psychotherapist will have dealt with the common trauma that children experience when they enter their parents bedroom unexpectedly and discover their parents having sex. Psychotherapists call this the "primal scene" and it is a common source of trauma and complicated feelings in children.
Young children do not understand adult sex and often mistake the sounds and sights of the rival scene as an act of violence.
Re the term "sex positive", I am supportive of adults feeling free to express themselves sexually, to feel able to experiment and play with partners to discover what their bodies like, without judgement, providing it is consensual of course.
I could be supportive of the "sex positive" movement if it was not so infiltrated by cults and predators.