How soon is too soon?

So guys.... my Mom and step-dad (married 16 years), are getting a divorce. He cheated on her about 4 years ago, she told us, but she stayed. A couple of months ago, he cheated again and things got crazy so they’ve been separated ever since. Now, here’s where the title kind of plays in.

Now, as you can see, my step-dad is the reason for all this because he broke up his family (there’s 5 of us kids, 3 still in the house). Anyway, my Mom has been talking to this guy on FB for the past couple months, like right after they separated. About a month ago, she went and met him. Now, not even a month later, he comes and STAYS, not visit, but STAYS long periods at my Mom’s house. I have a problem because my siblings live there and she literally JUST met this man. Ever since he started coming up there, she has my siblings cleaning like crazy and adjusting their lives because of him. She tells them they’re going to start going to church, he’s going to be checking their rooms and bathrooms to make sure it’s clean, have them workout, basically “Dad duties”. Plus, when she goes to the grocery store, the majority of the food she gets is based off what he likes.

Personally, I think this is insane and that he has no business being in the house or changing my siblings life around for someone who my Mom is just dating. I am 22 and married (husband is in military), no kids. But, I would not bring a man around my kids that I just met and have him doing things their father should be doing. I am not against the whole keeping the house clean because my siblings are teenagers and kind of messy, but he is NOT their dad. She’s told them he’s not trying to be their dad, but come on y’all, what does all that sound like?!!

I want my Mom to be happy by all means, but she is moving wayyyy to fast and bringing my siblings into is a big no no for me. She thinks I am being real judgy about a situation I know little about but my siblings tell me everything because she doesn’t. She even has them hiding things from my step-dad which I feel shouldn’t be something a child has to carry. I do not agree with this situation at all and feel it’s way too soon for him to be staying at Mom’s and meeting my siblings. What do you guys think? How soon is too soon?

PS. Sorry if it’s confusing but I can’t write everything.