Please Help! Problems with my parents not seeing their grandchildren.?

This relationship I have with my parents is more than strained at this point. My sister and I live in the same subdivision and my parents stop by and see my niece and nephew all the time. We live 1 min up the road. Sometimes they will go over there in the late afternoon and stay all evening and then come by my house for 15 minutes and by then it is too late and the my sons are in bed. This has been going on since we moved here like over a year ago. We visit them sometimes, but we stopped because they never come here. My sister never visits them. It makes no sense. She is older and has alot of near by family. My husband and I dont. The also babysit my neice and nephew alot. When I ask them they say they are going out and I never leave my children or go out to dinner alone with my husband because I have a son with Autism and my younger son has seizures. It hurts me. The last straw for me was the other night my parents power was off from a storm and I had invited them over.... well they ended up at my sisters house and the only reason I knew was because my brother was riding his bike in front of my house and he said that they came over to give my neice and nephew money. I called my mom and dad and told them they could see the grandkids whenever but I no longer wanted a personal relationship with them. Was that too harsh? What should I do or how should I handel this? I have brought this issue up at least 100 times. Please give me advice.