First of all, I have a 2 yr old and a 3 yr old. When they are in the bath, they are rowdy and love to play. We try our hardest to make sure they stay seated, but they love to jump around in there - we have to remind them and/or discipline them several times a week for wild behavior in the bath. My point is that I have had to coem to terms with the idea that even under my very watchful eye, one of my rowdy toddlers could have an ACCIDENT and fall and break his/her leg. It can happen to anyone. You CANNOT make this poor 9 yr old child feel as if it was her fault that her bro hurt himself. And by extension, it's not bio mom's fault either. Could have happened just as easily if she was watching.
I actually think it's pretty awesome that this other woman has her older children involved with the yunguns in this way. It will build character, responsibility, not to mention the family bond. Also, anyone who comes from a happy blended family will tell you no distinction is made between "full" and "half" siblings. And you can't dismiss family bonds over a large age difference. When the 15 yr old is having children of her own, her younger sibs will maybe be old enough to watch and/or play with them. One of our nearest and dearest relatives is my husband's aunt who helped raise him. She is 12 yrs younger than his mom, but 10 years older than he is.
But, back to your case. I sense that given the girls' school district, a judge might want to leave well enough alone. He may give some weight to the 15 yr old's preference in terms of visitation, so be careful about opening a can of worms there.