Someone had a real good post on this a few months back and I just can't find it (looked up Venus, Serena, Williams, tennis, sisters - no luck). The implications were their father got paid to make his sons into daughters early on in life and train them to compete in tennis. If I ever come across it I'll send the link.

Edit: If you look at some of the very early pictures of the kids you may get an inkling they could be boys.

https://duckduckgo.com/?q=venus+serena+father&t=h_&iax=images&ia=images&iai=https%3A%2F%2Fi.dailymail.co.uk%2Fi%2Fpix%2F2017%2F06%2F14%2F17%2F41661FCA00000578-4600658-image-a-10_1497458192311.jpg

Someone had a real good post on this a few months back and I just can't find it (looked up Venus, Serena, Williams, tennis, sisters - no luck). The implications were their father got paid to make his sons into daughters early on in life and train them to compete in tennis. If I ever come across it I'll send the link.