Higher but... Although genetic factors definitely play a role in autism, the exact risk for various scenarios can be hard to pin down. Having had a child with autism definitely increases your risk of having another by 10-20 fold. Having the family history you describe probably increases the risk too, but less so. Genetic counselors can give you the very latest numbers, but there's no reall way to prediction.
You need to know . The results of your family members' genetic tests-comparative genomic hybridization & dna probe for fragile x & a 3-generation family history on both your mom's & dad's sides for a geneticist or genetic counselor to answer your question. Ask your ob-gyn for a referral after collecting the needed information.