Boys being boys or Serious problem

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Now this isn't about my own children, but someone elses' child who is currently living with us. She's a newly single mom and apparently her parenting skills were along the lines of "let them do whatever they damn want" I run a pretty strict household. No running, screaming, limit whining, no TV during the week and so on. So she's surprised on the fact that my kids listen to me when her kids...well, don't. Her children have also grown up in an abusive home, so they think it's 'okay' to hit mommy. but they don't like other people yelling at mommy. So that's a little bit of background on this woman's kids. Now, my kids are older, but of course I'm pretty protective of them. I'm getting slightly scared for my kids because of her own kids. Her eldest son 'may' have slight autism. It might be genetic because her niece and nephew BOTH have autism. She, herself, doesn't understand complicated things and easily forgets things she did yesterday and she can't problem solve on her own. We don't know, but we feel she had a learning disability as well but found a way to 'function' in society. This kind of started the other day. Her son has a morbid sense of humor. We have a neighbor who has 3 cats who like to sit on their window. My son LOVES cats so he likes to watch them. Well, he son saw this and started (while laughing) saying he wants to eat the kitties. Okay, I let this one slide. In some places they do eat cats. So a few days later, my son and him are saying how much my son wants a cat for a pet. And her son smiles and said, "I want a kitty too, I wanna break it's legs" then starts laughing while motioning breaking something small with his hands. Okay...now I'm a little scared. We old found out that because he was mad at his little brother, he bashed his 2 year old brothers head into a wall. So hard, that 2 months later there is STILL a giant bruise on his brothers head. She didn't think anything of it and didn't even seek medical help for her youngest son. So my question is... Am I just being paranoid about this woman's kid with him just being a boy and saying things he doesn't mean. Like when he laughs and thinks it's funny to kill someone on a video game. Should I classify it as Boy's being Boys or is there some other serious problem I should look out for?