As long as she is kind to them, Ms. Palin should be allowed to support her children however she sees fit. Whether the child with Down syndrome is her's or her daughter's is nobody's business but her family's. The fact that her daughter is pregnant is also nobody's business but family.
My mom was undergoing chemotherapy for lung cancer when I called to tell her that I was pregnant and going to be a single mother. I apologized for causing her stress during an already stressful time. She replied firmly, "You never have to apologize for a grandchild of mine." That statement meant more to me than anyone can ever imagine.
Regardless of her politics, it sounds like Sarah Palin is being supportive of her daughter and her grandchild (grandchildren?). I respect her for that and wish people would stick to the issues and leave her family out of it.