Interference from family members

Why people think they have the right to interfere in others matters. I have 2 kids both Autistic. So my BIL and SIL told my husband that they think our 3rd baby will also have Autism. They never tell me these things because they know I will put them in their place as it is not their business. So basically they are trying to influence his decision when it is our decision to make together. Let me tell you about our so called third baby. He or She hasn't even been discussed, we have not decided if we are definately having or not having any more nor did we ask their advice on the matter. They are not Doctors nor Specialists but spend a few mins on google and pretend to be experts in everything. We are not stupid, we know the percentages of another baby having Autism as we did our research before having a second child.