@Vofuro True, but as it turned out, the risks were even bigger on the route they did choose. If Leslie wasn't around at all, they could have upheld the integrity of the family a little bit better. What comes to what
@spandxltd said, they could have just stated the marquise was pregnant, but the baby was stillborn or died shortly after if they didn't want to ensure the midwife stays silent. I can't imagine they kept Leslie on the record in case Eli dies for some reason, haha. With how Leslie was treated, it's hard to imagine she could have taken Eli's place as the proud daughter of the (rotten) Sperado family.