I'm not sure what the problem is..

My wife and I have been together for 5 years. We got married 2 years ago. I have 2 kids from a previous relationship. I have my kids 2 days during the week, and every other weekend. They're good kids, and keep to themselves. They're both early teens. Recently my wife had been pushing me to not have my kids those two days of the week.I told her I didn't want to do that, because that's my time, and they like coming over. It would hurt them. She's chosen to take the hands off (nacho) approach, so far there haven't been any issues. We both work, and whenever the kids are around she just pretends that they don't exist. Recently, she's started to get anxious before they come over, and is in a foul mood from the night before they get there until they leave. Once they're gone it's like she flips a switch and is amazing again. I've brought it up to her, and she says things like they don't listen or they're disrespectful, but I'm not seeing that. I may have blinders on, but if I ask them to do something they do it immediately. The younger one may need to be asked twice, but they never talk back or anything crazy. They pretty much just stay in their rooms unless we're going somewhere, and then I always take them. I always cook dinner for them, and take them places if they need it. My wife doesn't have any kids of her own, and we don't want to have kids together. I'm done having kids, and she's never wanted kids. She's never left alone with them and doesn't go anywhere with them. She's free to do whatever she wants and the schedule for when they're coming has never changed. I'm not sure what the issue is, and I'm wondering if anyone here has had a similar thing, and can give me some advice on how to approach the situation. I love my wife and I love my kids, and I just want us to all be harmonious during the small amount of time we all co habitate together.