[blam]

I have double 1st cousins, two sisters married two brothers. Unfortunately, my cousins were all afflicted with Marfan Syndome.

Me and my brothers and sisters are normal.

[Varda]

I understand that Marfans is a dominant gene. so you'd get the same effect if one sister married both brothers.

[JohnBovenmyer]

IIRC my mom had two first cousin sisters, who married brothers. Alas one of the brothers murdered his sister before my time.