Well it's true that the dangers really go up when the generations keep it going, but they are there even with the first time.
Basically everyone should be careful about who they breed with. If both parents have a chance to be carrying the same genetic disease they are basically gambling with their child's health. Bad parenting before the child is even conceived.
With incestuous breeding the chances go way up. Especially if future generations just keep it going.
However this applies to direct blood relations.
Breeding your hot step-mom is not anymore dangerous than any stranger. Unless of course she is your biological mother's sister or in some other way blood related to you.
It's not biologically dangerous, but good grief there is bound to be something wrong with the backstory of that relationship. It's adultery at the very least.