You are definitely in a tight spot. Hmm... there are a couple of ways to approach this, but either way, YOU are going to have to make a decision. Either you want this to change for the better or you don't. What has happened so far is in the past and unless you want the future to be the same, you're going to have to make a committment to behave differently.
You're the adult and they are the kids, but which is acting like which? It is your responsibility as the adult and the mom/stepmom to set the standard. If you want to be treated badly then I'd say you're headed down the right path. If you want to be treated as a jewel in your family's crown, then you need to give all the children in the family the same treatment that you expect.
Perhaps you see the kids as an intrusion to your relationship with your husband, but if that's the case, you're pouting and need to change your frame of mind. You knew he had kids, and it is NEVER right to make a parent choose their kids or their new spouse, and it's not right for you to shun his kids like this. Maybe you don't realize that you don't have to "love" his kids to make them feel like you do, and that WILL be the best thing in the long run. You don't have to change your mindset to change your behavior. Change your behavior and let the behavior change your mindset.
In the meantime make it a point for you and your husband to have "couple-time" at least once a week. You are still newlyweds and you both need to prioritize your marriage. You don't have to go out (if you can, that's even better) but set aside a night to have couple-time to JUST enjoy each other. This would not be the night to talk about the stressful day, broken washers, etc. Talk about what you like about each other, talk about sex, eat some fruit with whipped cream and enjoy each other thoroughly.
You could also be afraid of getting close to these children, and if that's the case you're letting fear rule your life. Maybe you think if you allow yourself to really like and maybe even love these kids, it will really hurt if they don't feel the same way. Maybe you don't want to go through the hurt and sadness you know will be at the end of those 3 weeks when they go home.
Rejection doesn't hurt as much when you don't let them in and/or you are the one doing the rejecting. How committed is this marriage? Are you both truly committed to being married for life? Are you 100% sure of that 100%-life-long committment? Maybe you're afraid that this marriage won't last and somewhere in your mind you think you're saving yourself from some hurt by distancing yourself from the kids. I can tell you, you're not saving yourself from hurt, you're causing it and not just for yourself.
It could be that you've gotten the "Wicked Stepmom Syndrome" into your head, and without even realizing it your letting your behavior slip right into what you subconciously think those behavior patterns are.
It doesn't really matter what is causing this behavior. What matters is that you change it, period, no question you cannot treat these kids like this (his or yours) and it not disrupt your entire family. It is your job as the mom to bring your family together and create an atmoshpere of love and acceptance. I doubt any of the kids are feeling that.
Being a woman of grace is not always easy, being a mother of grace (love, joy, acceptance, peace, comfort) is a challenge, but being a stepmom of grace is a true testament of the word challenge and the rewards of meeting that challenge will change your life and lives of those children for the better.
