genetics :/

andrea • pregnant!!! due July 3rd 2016 with baby #1!!
my fiancé and I have been TTC for one year. however i have PCOS (that's a story for a different post) but for now I have a question about genetics so my fiancés older brother is autistic and on my dads sign of the family we have a history of deformities and mental retardation. so far it has been every 3rd child my grandmothers younger sister (3rd child) her youngest daughter (3rd child: my dads sister.) she passed away when she was 12 and my dads other sister has 4 children and the 3rd one is also disabled. she is 8 years old and she was born with half a heart, no spleen, her stomach is in the wrong place, she has had 3 strokes, she is paralyzed on the right side, she has a G-tube(feeding tube), her arteries and veins are the size of an infants and even on large amount of blood thinners eventually her artieries and veins wil be unable to support her body. long story short: my parents only had me in fear about genetic diseases. 
​I was wondering what the chances are of my child having a disability since my fiancés older brother is autistic and my family has a history of disabilities. 
​thanks in advance baby dust to all.