Although karif has ignored me so far, I'm going to try again. This sounds way beyond Sensory Integration Disorder (SID). My son had Sensory Integration Disorder (SID) and this is violence. That doesn't mean she doesn't have Sensory Integration Disorder (SID), it just means that in my opinion it's not just Sensory Integration Disorder (SID).
I again say, this child was exposed to drugs and alcohol. He could have fetal alcohol spectrum effects. This will NOT respond to medications or behavioral therapy and it is beyond autism (Autism Spectrum Disorders (ASD) kids improve with interventions). I am surprised karif won't address this--maybe this is too scary for her to contemplate, but with any child who was exposed to substances in utero, it is a common problem.
These are some of the most difficult children around because they don't understand right from wrong, and no amount of teaching can explain it to them. They need a special kind of help. This baby deserves a safe home, and he doesn't have it. If your difficult child is hitting the baby even when you hold him, I wonder if he is capable of living in a family environment. He is obviously not correctly diagnosed or treated, and does not learn from his mistakes and medication hasn't helped him. This is on the extreme even for kids adopted from foster care. It's true that you CAN lose your children if this baby is bruised. And CPS may decided YOU bruised the baby, rather than the difficult child. This is a hot potato. I'd do something fast. I'm not sure what you are open to doing, but I'd find out if Fetal Alcohol Effects (FAE) is an issue here and then discuss realistic options for this child. He may need to be placed elsewhere where there are no other children in order for you and your other kids to be safe. A friend of mine had two foster girls and one was much like your difficult child. One night, she tried to choke her foster sister. The foster sister could not scream for help and when a brother happened to pass by the room and saw it, by then the foster sister's neck was bleeding and red and full of welts. Everyone in the family, including the older kids, were questioned. The violent child was voluntarily removed, although they were going to adopt her. She has been through at least six placements since then. Some kids can not be saved in spite of our best intentions, and if you have other kids, you have to think of them too. If you want to parent this child, you need a total neuropsychologist evaluation (maybe once a year) to see the total picture of this child, and you need to face everything that may be wrong with him so that you can keep your other kids as safe as possible. Ok, I won't harp on the fetal alcohol part anymore, but..you seem, for some reason..not to even want to "go there" even though it is a distinct possibility with this child. Most parents here will not think of that becauce they did not adopt kids at all, let alone that were exposed to substances in utero, but, hey, I have. Good luck.