I didn't know my first child was autistic when DH and I decided to have our second, but we did realise he had special needs of some kind.
Yes, autism is partly (and - in my personal opinion when it's an Asperger's type profile - largely) genetic. My second child is also autistic.
I'm not going to lie, the challenges are big.
But, DH and I are so grateful we had our second child. They play together, they each have someone in their corner (albeit two quirky corners) and our lives are all richer.
Our second child has more complex needs than our first, even though on the surface she looks more typical than her brother. Her anxiety has a huge impact on her daily behaviour.
You don't know what needs, extent of disability, personality, strengths etc your potential second child will be born with. You have to be able to look the reality squarely in the face that you can't control who your child will be. So you make your decision based on whether you want a bigger family than you have now, and if you have the resolve to nurture your bigger family whatever that looks like.
DH and I are tired, strained, up against it a lot of the time especially in these early years, but so far I still have a resounding "yes" in me about our decision to have a second child.