We have four kids 4 and under, and we all sleep in one bedroom together. What do you mean by family-friendly? Just pretend you're one of the millions of big families living in India in a small house :) People do it all over the world.
Our biggest concern is making it adult-friendly. We put the kids to bed, except the baby, around 7:30, then we hang out in the kitchen for an hour. We don't have a living room, it's just the kitchen and the bedroom.
Then when we feel like going to bed, or when the baby is too cranky, we go to bed. Our other kids (twin 3s and a 4 year old) are good at sleeping through noise. I imagine your twins are that way too since they're used to sleeping through each other's cries.
So, we quietly go into the bedroom, get undressed, go to sleep or have sex, and we don't wake up the others! I mean, we really can be loud. We can turn on the light, we can have normal voice conversations, and they just sleep sleep sleep. Also, if someone were to wake up, we just say go back to sleep, and they do. There's no problems since you stay in the room with them, they don't feel abandoned and start crying.
I'm not sure if I'm getting at your concerns. Could you be more specific about which part you think is unfeasible?