I agree they know full well when you are trapped - that's when they go for the jugular, knowing there's not much you can do about it.
I used to wonder at the number of times a poison arrow (or 10) came my way at a time I couldn't say anything - usually someone's special event: birthday, wedding, anniversary. Always a public barb, with me stuck there like a stuck pig, unable to respond or I'd 'spoil' the party.
The solution is to not be there.
When people like this get you in their web, you kind of go to sleep. You become uber powerless, overwhelmed by the onslaught. I used to be so fearful of 'saying anything' I'd literally go dizzy when the opportunity presented itself.
I've often said on here that the abuse rendered me speechless: I wouldn't know where to start. Everthing I wanted to say sounded so lame - and, anyway, I knew a stinging attack would come winging back in record time if I dared try.
The solution is to not be there.
So, OP, why do you keep going back into the lion's den? Really, genuinely, why do you go back? I'm not pointing fingers - I've been in exactly your situation. I longed for a good family and that's what kept me going back. I hoped and hoped, against all reason, they'd be decent. They never were - and, crucially, they never will be. Their sport is jabbing at you, mine at me, with pointy arrows to draw blood. They like it, they enjoy it, they get off on it.
Sad hey 