Not if they contribute meaningfully to their siblings children's survival. Your argument also assumes that homosexuality is primarily determined by genetics and/or that they didn't father or bear children in spite of their orientation.

The part where I said "non-reproducing people" and "(if you believe the "genetics" argument)" cover what you said.

You refute your original argument with your first sentence.

To contribute meaningfully to their siblings children's survival would normally take place before they are 40.