Between Jane and your mother, most sensible solutions are closed to you. Unlike many people here, I don't think you should "not all men" her anyway. It's rarely useful and demonstrates a mis-alignment around the emotional damages being discussed. That is, the total misery inflicted on group A by some members of group B wildly outweighs the smaller misery felt by "nice" members of group B when they are lumped with the not-nice ones, so the point is not a useful one to make.

Useful points to make abound, including that you don't need to be lectured on things you already know, that you are happier discussing other topics, and that she is upsetting your mother. Yet you are constrained from making any of these. So, what can you do? You can vote with your feet. When the speech starts, don't engage. Try something like:

>I am sorry to hear that this is troubling you again. I apologize for any pain I've just reminded you of. If you'll excuse me, I need to take care of something.

And you get up and walk away - to the bathroom, or to step outside, or to another room and another group of people if that's feasible. You wait a minute or two (longer if your mother is not being subjected to the speech in your absence), and then you return. It is unlikely the speech will be ongoing in the absence of anyone to argue with, but if it is, you can simply hold back from rejoining the group. If your mother appears trapped there, you can try the old

>Mother, can I see you in the kitchen for a moment?

And once there the two of you can discuss whether she is ok with staying in the conversation with Jane or having an excuse to leave it.

I am not claiming that over time this will be some sort of aversion therapy that will teach Jane to change the subject. I suggest it only as a way to get yourself out of an unpleasant situation simply and without lying or arguing. (You're **not** lying: the something you need to take care of is your own well being. You do that by going somewhere you can be alone, or at least away from Jane.)