Please help and advise
Honestly no idea where to start..
My now husband and I came together out of an affair. He was single, I was married. I left my husband for him and I knew his 3 children before we all moved in together. I have a son who spends half time with me and half time with my ex. We have all lived together for over 3 years and we got married last year. It's been hell with his kids ever since we moved in together. To be honest, it was the thought of his kids that kept me in the affair as I knew leaving my husband for him and his 3 kids would be a challenge. But I love my husband so much and my relationship with him, for the most part, is amazing. He has his kids roughly half time too. My son and his kids have always got along amazingly well which is great.
There are a couple of days his kids come here when my son isn't here and it's hell on earth. Truthfully, I hate it when they are here at any time, but it is easier when they are here with my son at the same time. I cannot love them, and nor do I like them, I tolerate them. They are rude, disrespectful, loud and obnoxious all the time and constantly blame me for instilling rules and discipline in our home. My husband used to let them get away with murder when he lived alone with them and now they blame me for 'the changes'. When I'm showering them with gifts and doing what they want it's fine. My son is so quiet and gentle and quite frankly perfect.. yes, I know how that sounds but I've raised him strictly and it's paid off. I don't want to rant on but things have got so bad this is having a serious effect of my relationship with my husband. He knows I can't stand his kids and I feel terrible as I tell him a lot! I was so naive to think this could work right? I dread his kids coming here and how fickle they are. I hide in the house a lot and feel my blood pressure raising as they cause mayhem. More stuff has happened but it's lengthy. I love the time I spend with just my son or the times my husband and I have alone, but these moments aren't enough to make me feel happy overall. I don't want to leave my husband but my hatred for his kids is killing him and their being in my life is killing me 
Advice please
I feel as though I could have
I feel as though I could have written this post! I feel very similar to you...perhaps even more extreme because I do not allow the skids to come to my house anymore. It's been well over a year. But it's still not good enough. I hate the situation that much that just knowing it exists is too much for me. I feel so angry about it and the years I have been involved in it that when I contemplate leaving DH, it's not enough. I want BM, skids, DH etc to all know how miserable they made my life.
As far as advice goes: I think many of us here just don't want to accept that the only way to happiness and peace is to leave.
So many of us talk about how great things are with DH and how well we get along without the skids. For me, I just can't live with the fact that I came second and I am last priority. I can't understand DH having love for skids or even BM. I want him to hate them....that's my issue. They have destroyed his life and he has no choice except to be stuck in it. I resent them all for financially sucking my husband dry and for making him feel inadepquate and depressed. He should have never been a parent. It's way too hard on him and I love him so I want to protect him from the leeches. But the law says he has to be in this mess until they're both 18. 6 more years. DH talks about suicide all the time...or moving far away to an island. I feel bad for everyone involved. I wish we could all acknowledge that this isn't working and wipe our hands clean of each other.
I mean really! BM and DH couldn't make it work together. And then child custody arrangements force them to be in each other's lives til the skids are adults? That's insane. I wish DH and BM were smarter and could just make their own arrangement and largely be uninvolved in the other's life. They still have each other in a choke hold and BM is just angry. I would be too if I got stuck with her kids.
There's my answer. Writing is on the wall and I've known that I should leave but I don't. I just count down til age 18 and think that will magically change something.