"That sex was was great sis. Your vagina is like a mares." Realising what you said, you saw your sisters vagina change. Her vagina started to grow bigger until it reached an large size. Horse hair started to grow around it and her pussy lips started to turn black with a similar coloration to the black horse hair around it. Inside it her ovaries grew to the size of a mares as well as all the other reproductive organs in her body.
Your sister then said "well of course it feels like a mares since it is a horse mares vagina. It has the every organ used in a reproduction but each one is the size and shape of a horse. I though you knew this already and that's also why we have to perform incest to reproduce. I do hope the children will be nice and strong like us."
Hearing that made you realise that what you say can also have effects on reality. You just thought that maybe your entire family has horse cocks and cunts which scares you slightly on how powerful you really are.