C.M.
I would absolutely NOT leave it alone. Isn't their life hard enough having gone through divorces that now they have someone telling them who their family is? That's confusing and cruel.
I would call her and tell her that it is not necessary for her to explain to the children that their brothers and sisters are half's... and guess what... they are REAL brothers and sisters... they all share the same mother...my god.. what if you adopted... she'd really get into it...
I wouldn't worry about stepping on her toes... she just stepped on your foot!
Again... just tell her that you don't' want her explaining it any further. If she gets worked up over it... then that's her problem... if she's such a great person... then she'll just say something like "wow... I'm sorry...I won't do that anymore."... not defend HER point of view over YOUR children...
good luck...