These things don't happen in a vacuum.  WHY are your husband's children moving in?  Is there a problem in their mother's house, is she moving away, or did they just feel like switching homes?  Were you consulted at all?  The mere fact that they're moving in doesn't mean you're not your husband's priority, unless it's a completely voluntary move that you vehemently protested.  Any time you marry a man with minor children it is a  possibility that they will come to live with him, and while you are his first priority, they are his first responsibility.  

Will the mother have visitation?  What kind of schedule are you looking at?  It may not be as bad as you're anticipating.

Futuro makes a good point - the children are your husband's responsibility, and particularly if you were not in favor of this move, he needs to do the heavy lifting with respect to the kids.

I agree with futuro! If it means that you now have some free weekends, that sounds like a deal to me!

During the week with school and sports, kids are busy. I feel like I barely actully spend time with mine during the week. On the weekends is when all the "quality" time comes in. If you only have to play nice 2 weekends a month-- not a bad deal!

What about summer and school holiday's? Do the go  to BM for those?

Yes. More context, please. Why are they moving in? 

I also would never want my skids to move in with me (they're all grown now and I dodged that bullet, thank Dog.) But. If you're already used to having them every single weekend this might not be as much of a change as you are anticipating. Assuming weekends were Friday after school - Monday morning, that's a solid 12 nights / 13-14 days a month. Will you reverse that and BM gets them every weekend? 

One thing you should let yourself off the hook for = your feelings about it. Being a SM is a completely unnatural position to be put in, and it brings up strange, sometimes awful feelings. I, too, often beat myself up for how I felt about my skids over the years. Then, one day, I took a step back and looked instead at how I treated them and acted toward them. If marriage counselors, pastors, therapists, and others are correct that love is not a feeling but an action...then I was loving my skids. I decided to forgive myself for my feelings...and also tried to locate the source of those feelings. You're ahead of the game because you already realize your skids are basically fine and that the source of your anxiety, resentment, and general bad feelings are the result of not being prioritized or consulted in your marriage. It took me a little while...and some self-reflection...to realize that fact and start to address the actual problem instead of displacing it onto the skids.

I recommend that you start prioritizing yourself immediately, even if you don't do anything else or want to hold off on making a decision for a while. I did this, and it made an enormous difference for me. I claimed a room as my office and made skids share a room. I started paying myself first out of my paycheck to secure my future. I started making foods that I like instead of always thinking about what DH and his kids like, I started insisting on more time with my family...small things that made a huge impact for me. Is there space in the home you can claim for yourself? Is there some time you can carve out for yourself? 

It sounds like you don't have any kids yourself. Is that right? Do you want to? Are you worried that if your skids come live with your full-time, your DH won't want to start a family with you...will expect this to be your family? 

Lots of questions to answer. Sorry. I'm a childless SM myself, and I think that can be an even harder position to be in. 

I'm in a similar situation - DW wants her two kids to move in. This is problematic. It wasn't on the table when we got married, but now she wants them with us. It feels like a unilateral decision from DW's side, and also that she's ignoring my interests and needs. In a way she's choosing her kids over me, so it's natural that you'd feel a little betrayed, even.

How you are feeling is perfectly normal. As others have said, more context is needed as to why they are moving in. My skids unexpectedly moved in with us full time a year ago. It has been pretty awful for 90% of the time. If I were you, I would try to avoid it at all costs. Coming over every other weekend or for some of the time is very different to having them there ALL of the time. 

My 15 SD is moving in due to her mother no longer wanting her there. very disobedient...very disrespectful..

I had a terrible experience with her over the summer. I have 3 kids and im pregnant and my husband is telling me if i dont want his daughter here then he will get his own place...im crying on the inside bc i dont want to lose my husband or take him away from our kids. I feel i have no choice in this matter. i almost gave up and told him to just get his own place but i will miss him too much. I am going to fall in a depression over this. I cant imagine her living here with us. my house is already ful land now i have to see her everyday. i feel like this was a stunt by her to get her way. she probably behaved so badly to the point her mother didnt want her there. my husband is saying i can discipline her but DAMN!!!!, I dont want to go through this ...i dont know how this will end. i feel she will end up tearing my marriage apart. I dont know what to do. Im hating i allowed myself to marry someone who has kids. In my head I think, do I lose my husband and allow my kids to grow up without their dad or do I suck it up and deal with his daughter....