One of the hardest things about being a part-time stepparent is dealing with your emotions while the stepkids are at the other house. Say, for instance, you find out that they lied to you, your spouse, or your family members. For some reason, we tend to find those things out when the kids are not with us. Then you spend the entire week or even 2 weeks thinking about it, fretting about it, and talking about how to handle it.... when they come back. To be healthy, you're supposed to deal with things, confront issues, and talk about what needs to change. With children, you need to correct behaviors when they happen and discuss options, solutions, or penalties at the time of the event. But in this situation, you cannot. You're left in a holding pattern.
This even presents itself in small ways all the time- not always with big rule breaking or offending behavior. Sometimes it's just that they didn't do something you asked them to do repeatedly and had incorrectly assumed they had done. Easiest example of that is cleaning their rooms, putting away dishes, finishing a task. I often discover many of those types of small things were not done like we asked, but now they're gone and I can't remind them for possibly up to 2 weeks.
Then you have choices to make: Do you just clean it up or finish it right then because it will seriously bother or annoy you for the entire time they're gone, make them do it when they are there for only a short, rushed visit, make them do it the moment they walk in the house after being gone for up to 2 weeks, or make your spouse do it- since they're really his kids anyways...?
Of course, there are some things you have to do immediately. That is the most frustrating, of course. You are left only one option: do it now, or ants will take over your house. Do it now, or you will trip over it, like you told them when you asked them to pick it up. Do it now, or the dog will chew it up. Do it now, or you'll be embarrassed when your parents come over to visit. That's the worst. You feel like you were pushed into a corner by unwitting, absent-minded children. Normally, if they were there, you just yell up the stairs "Somebody left their socks down here!" or say "You forgot your plate"... Mission accomplished. But when they're gone, you're the maid or you have to deal with the mess.
And that's how resentment can also grow- When the kids aren't even there!
You could come up with punishments, make them do more when they come back over, or save the work until then. We try to do the latter, but usually we're simply left with telling them that they left a mess for us and that I had to clean it all up because I had no choice. Let me tell you- that has absolutely no effect on kids. They weren't here to see it, witness it, hear my grumblings, or clean it- means it didn't happen. Their world is here and now. Occasionally I see my youngest stepdaughter feign sorrow over such events, but it's primarily because she's super empathetic. She just wants me to be happy and be happy with her, but she still will not catch whatever it was she was supposed to have done. Hence, the child's understanding of responsibility.
Inevitably, one parent is less OK with dealing with the problems from last time when the kids come back. Usually the birth parent, of course. They finally get to see their kids, so they are uncomfortable or worried about dealing with the "dirt" the moment the kids come over. But if it's not dealt with, stepmoms feel more and more like maids. It's a trade-off- help your kids learn responsibilities in their "other" house, or let your wife feel like she picks up after your dirty children.
So we eventually came around to making them put away things the moment they had time or having them clean up, if it was able to wait. When it can't wait, we do it...and sometimes we discuss repercussions. Usually it just materializes the next time they come over in more strictness on those things that were left undone the time before. Then they're not being punished, but the expectations are made loud(er) and clear(er). All in all, the more we're on top of it all, the more they're on top of it. They reflect what we make priorities. We tell them that they will not go to their friend's house if their room is not cleaned, and it better be more clean than the last time! They will need to forget that movie they begged to see if they can't pick up after themselves..etc. So instead of punishments, it's more of a follow-up code enforcement the next time they're over.
It's much harder to deal with the bigger issues, hurt feelings, or ignoring during the off-time, though...