IMO, this has nothing to do with any actual damage Ms Hairdog could or would do to the children's college fund. These are not Ms Hairdog's children, and she is not responsible for their education. While it would be pretty to love the children of one's spouse as dearly as one's own kids I don't require pretty.
She did, however, marry a man with (three?) prior children, and it is only fair that a portion of hairdog's money goes into the college pot before being added to the general household pot. If she wants to contribute her own money, great. If not, fine. It is, however, unacceptable, to use that contribution as a threat in an argument. I don't care if she doesn't really mean it. I don't care if the actual ramifications are negligible. I don't care if hairdog is a great dad (I'm sure he is, and this has nothing to do with his children's well-being. This is all about Ms Hairdog). Contribute or don't contribute, but do NOT use the kids as a tool. Her behaviour is unacceptable, and if you let her get away with it she will perceive it as a lethal weakness. I would.
