my family suck. they really do

ok here it is.. I grew up in a family that would be anyone's worst nightmare. we were exiled from our extended family because when my mum was younger she moved in with her sister and slept with her husband, having to children to him. I am the middle child of 3 sisters. my sisters have the same dad but i was created through a one night stand so i look nothing like my sisters and am totally different. My sisters dad molested me repeatedly when i was just 2. My mum new but did not care. I had forgotten about it till i was 12 and started having dreams about it. when i asked my mum she called me a lyre but later claimed that the same man, being my sisters dad and my unties husband had also abused her as a child. I had therapy and went under hypnosis and now i remember everything. my sisters asleep in there beds my mother walking past the bedroom door, the way the room was layed out even the house and its furnishings. later in life my older sister confessed that she too had been abused sexually. Our mother also abused us but in a different way. she would tell us we ruined her life and she should just kill herself. I left home early at 14 and moved on... my sisters still talk to our mother but I have not spoken to her in 15 years. I send her photos of my beautiful children but i do that just to hurt her because I know it would. My older sister has turned out the complete opposite to me. she is on welfare, I own my own business. she is very obese, I am healthy and play lots of sports. she is very sad and bitter over our horrible childhood and i have used it as a tool. a learning of what not to do or say to your children. my sister now has 2 children that i believe are suffering from environmental autism. they are 2 and 4 they don't talk and are afraid of people. they hit ,bite and spit at each other. If i say anything my sister will never let me see my nieces so i say nothing. her home is disgusting i only visit on hot days so our children can play outside. How do I help my sister. she is blind to the joy of life and has bought 2 children into it to live her hell with her.. what do i do.. my younger sister is ok. she has never been in a relationship and suffers from depression but holds down a good job and owns her own home. she is a little strange but has a good heart and we get along well. its my nieces I wish i could help. I wish i could raise them myself and show them that in life you can be happy and joyful. I want them to dance and sing. smile and laugh but Its not my place and they are not my children... how do i help my sister