That's very honest of you and pretty hard on you as well. I am so sorry about that.
Are the children very young? Honestly, my children are older than my sister's and though I love her children like my own it was very hard for me to be around them sometimes because "I had already gone through whatever stage" with my own as many times as I cared too... Once they got older then it was fine. Oh God, the baby-thing was really tough.
I would love to have two children with my Grump HOWEVER -
We are both too old.
and though I wouldnt mind being pregnant (if I were healthy) I couldnt stand the thought of a newborn 24/7.... and then, MY GOD the pottytraining!!! I had to leave when my sister went through it with hers. Just too painful to even watch.
I suppose then watching my own children in adolescense (though admittedly they are in a TERRBILE situation) I imagine even *normal children* can be very taxing during this stage.