And for those of you wondering, yes, this CAN happen without dipping into underage content.
Alex is 18 when Emma is born.
Alex is 36 when Cindy is born.
Alex is 54 when Mindy is born.
Alex is 72 when Wendy is born.
After that you'd be stretching it to add another generation if Alex is always waiting for his daugthers to be 18 before impregnating them, because he'd be 90 when Wendy turns 18.