many sides to storyI've been thinking about your question for over an hour, and don't think I have come up with any firm opinions or ideas, and I'm not an expert, but here are my thoughts. I should mention we have a blended family situation, although my partner's children, now 18 and 20 (they've been coming to stay with us for 15 years) lived full time with their mum. My partner and I have two children, 11 and nearly 4.
I was babysitting when I was 12, but I think I was too young - I had situations that were beyond me. The legal age is 14 where I live. I wouldn't leave my younger children with a 12 year old.
I share your shock that bm suggested such a thing, and my first reaction was that it was OTT. Twelve year olds are still innocent, or hopefully still are.
Then I considered the fact that yes, they are not birth siblings, and that some children are sexualised that young. Also, I don't know your personal situation.
So yes, it could happen.
But I am assuming you and your husband are capable parents, and that you are intuitively connected with your children. In which case you would probably know if your 12 year old was sexualised himself and likely to pass it on to younger children. Are we talking about flirtation or something more serious? Parenting calls for intuitive calls of judgement all the time. Fear is not always the right guide for parenting.
If you are tuned in to your kids, then you intuitively know what they are ready for, what they are capable of.
The problem is, your husbands former partner has a different criteria. She isn't tuned into your children, doesn't know them as well as you do, and her only concern is her own children. If I was her, in that situation, I would find it very challenging. Hopefully, rather than reacting to fearful thoughts in a controlling way, I would raise fair and reasonable issues tactfully.
If she is open to reassurance, your husband could reassure her that your son is not sexualised, if that is the case, which I'm guessing it is since the suggestion shocked you, and that you as a matter of course observe the behaviour of the children, know instinctively what they're up to, and you can assure her there is nothing to worry about. Keep an eye on the situation, but in a way that preserves the childrens' innocence. Keep everyone's best interests in mind, including your son, including your partner's children.
If you are not sure, or have any doubts about the safety of the situation for either your children or your husband's children, then err on the side of caution. Listen to your intuition. Seeing my 11 year old and his friends, it's obvious who is sexualised and who isn't.
Having said that, I flirted with my cousin from the age of four til 14 but there was never any chance of it being acted on because we were none the wiser; to have interfered with our friendship would have been the wrong thing. That alas, was a more innocent age.
My partner's children from a previous marriage have babysat my children, with the oldest, the boy, babysitting from the age of 14. Some 14 year old boys would have been a bad choice for babysitting, but he was not. The main problem I found was that when they were younger, they didn't send my son to bed when he needed to go, and when they were older, the daughter was too interested in her own social life to do the job well, although the kids were still safe. But all in all, they helped us out when we had no money to spend on babysitters to alleviate the stress of parenting. It wasn't a bad thing. We would have pulled the plug if we'd found the situation detrimental.