I think the key question here OP is whether you & your DH can manage 2 children with zero family help? If the answer is no then I don’t think you should have another child & I would say this to anyone, even if they’re fit & healthy as no one knows what the future holds.
Your family are a great help & support NOW but that might not always be the case. You say your parents are older, what happens if their health deteriorates? Will your siblings still be able to help you if they also have to care for your parents?
Do your siblings have children? Does your cousin have children? What happens if they have (more) children & their child has some type of SN, would they still be able to support you then?
Are your siblings & cousin married? How do their spouses feel about potentially offering childcare? If they’re single, what happens if they meet someone who doesn’t want to offer care for your children?
What happens if your siblings / cousin move away or become ill themselves?
What if your second child has SN, would you be able to manage? Would your family be able to manage?
I’m 32 & have a chronic health condition which is why, thus far, I have decided against having children as I know there will times when I absolutely will need help & it’s not right or fair of me to expect this help to always be available, in fact it would be very selfish of me especially as the nature of my illness means I could have a flare up & need help / assistance without any notice.
Luckily I’m getting to a financial situation where if my family weren’t able to help, I could “buy” help, is that something your finances would allow?