So you taught her all about sex and giving head and then you were surprised when she went out and tried it?
I’d say that what happened next was pretty much all your fault dumbass!
Thirteen year old girls should be watching “Hannah Montana” not learning about giving head.
None of that even addressed the question anyway.
The mom asked him to let her know if he found anything, he said he would, he found stuff, he told the mom. End of story!
It’s not his responsibility to raise the thirteen year old (as it was not yours to raise your younger sister), it’s the mom’s. She should be allowed to handle it in whatever manner she deems fit.