I have a younger sister with autism, no one else in my family has it including extended family what is the likelihood i will have a child with autism?

Same as population: The cited risks in the general population run from ~1/60 to 1/88. The actual figures per person are hard to factor but in cases where no additional cases are within the extended family this is likely the best estimate. Newer research points to subtle changes in the patients DNA are found in an increasing number of studied cases. This change would be present at conception. More study is needed.

Probably <3%: Current stats suggest that the true incidence of ASD is stabilizing, as it has taken a while to have consistent diagnosis criteria across the country. Both genetic and environmental factors play a role. Currently ASD is more common in boys, with a ratio of 4:1 boys:girls, and the incidence of 1 of 38 boys or 2.7% and 1 of 152 or 0.7% in girls in children by age 8.