Losing a Mother In Law; One Upside to Divorce

So I have never gotten along particularly well with my mother in law. For those of you that do... cheers to you the lucky few. We have tolerated each other to death over the course of my 16 year marriage. I really can't stand the woman but she is good to my kids for the most part and has done a lot to help us with child care over the years... sometimes we have to pay her but that will be another post all together as summer vacation is right around the corner.

What do you do when your mother in law starts talking smack about you to your kids during the divorce? She watched my kids this weekend, asked for them actually then told my oldest "it's too bad your mom is too busy playing tennis on the weekend's to spend time with you." My 13 year old daughter had to defend me. I would like to point out the fact to her that there are exactly 168 hours in a week. 40 I am working, 5 hours on Sunday I am playing tennis and/or watching my team play. That leaves 123 hours most of which are spent with my children (I know some of that we are all sleeping but you see what I am getting at). I am there day in and day out. My soon to be ex-husband travels for work, he is self-employed and this career is by his own design. He is gone a min. of 2 weeks every month, often 3 weeks. In the last 2 weeks he has spent exactly 7 hours with our children.  I am here, alone, doing it all by myself, all of the rest of the time. (Hence the divorce, this does not make for a good marriage) When he is in town he does spend a lot of time with the kids but I still do all of the cooking, shopping, cleaning, shuttling around, etc. He helps with none of it. Is it really even worth having this argument with her or should I do what I have done for years and refuse to engage? I don't ever ask for her opinion but she does tend to offer it freely.

When is it time to tell your mother in law to shove it up her $%*???