MIL is driving me nuts!

Since moving to another country my SO his two children and I have been staying with his parents. My SO and FIL go away for work for months but try to make it home every other weekend, with a 3rd kid on the way we've been saving up for our own place and are hoping to be out on our own pretty soon. Well let me state my MIL was a teen mom when she had my SO she's 40 now with 2 younger kids a 3 yr old son and 1 yr old daughter. I understand kids will be kids and they don't really understand the concept or value of things but tonight after dinner I caught her son in our room messing with my SO's stereo and he broke it. Which I know would drive my SO nuts lol he was also messing with the electrical outlet now imagine how that could have turned out! Well my step daughter goes and tells my MIL and she completely flipped out on her claiming that if we don't want him in there to keep our doors closed wellll our door was closed lol and it wasn't my step daughters fault in the first place so I told my SO what happened and he wrote her a text and she did not like that she came barging in to let me know that if her son is such a pain to us then we should keep him out of our area and to not bother or interact with him. I mean I have no problems with her kids but seriously when you don't watch or correct your kids cause you're too busy on FB than there's definitely a problem.