Take the vacation with your kid, perhaps if he has to deal with them 24/7 he will realize what's going on and try to fix it.

Talk calmly with your husband about what is bothering you. Most bioparents can't accept their kids flaws, I'm not sure if they can't see them or they don't want to admit it because they see it as a failure on the parents part. Either way, address the problem, based on how it affects you & your son, with examples rather than personal insults at the skids. Does that make sense? If your sons truly being treated badly, I'd start with that. Say to your husband, I'm really concerned for our son, _ happened, what can be done about this? If your son continues to be bullied, I suggest you do stay somewhere else with him.

I would leave most of it up to dh except when they cross the line with your son - or being rude or disrespectful to you. Even though they are the skids, it does not excuse their poor behavior. Treat them as if they are your friends child - would you let your friends child talk to you in such a manner? If not, call them out on it. And I would also talk to dh about the manipulation - being a man (and their father), he may not even see it!

When my dh and I were first married, his 2 daughters would use tears as a way to manipulate him into giving in. Men are afraid of tears! Being a woman and not their birth-mother, I saw immediately what they were doing. After 4 years and much talking to dh, we no longer have waterworks from them. That does not mean that they didnt develop other ways to manipulate - they are children after all lol.

If they are terrorizing your son ( and I've been through that) then if you can pick up and go somewhere else, that would be ideal. Otherwise, he needs to either have his daughters within six feet of him at all times or you need to have your son within six feet of your at all times. Never is your son to be out of your sight. This way you can monitor all interactions, and bullying/terrorizing a younger child is justified reason for you to lay down the law and give them consequences. You have to protect your child. If they are bullying/terrorizing him, you have every right to step in. Would you ignore a kid being bullied in the street? Or would you tell the bullies to go take a hike? And if your DH protests (because they are going to have to spend the rest of the day in their rooms without their electronics) you tell him it's either this or he pays for a hotel room for you and DS for the rest of your stay. Which does he choose?

I wouldn't leave the house. This will make them continue this behavior because their reward is that you back down and leave. They win. Talk to your husband. Agree on house rules and consequences. This way, if they break a house rule, either of the adults can step in and give them the consequence without fighting with each other. You guys need to be on the same page. Kids do this to their parents all the time. They push the envelope to see how far they can go. They even do it in nuclear families.