My [M29] sister [F28] started having sex with me after findinf out I'm infertile.

My sister and I have always been close, she has five kids now aged between 1 and 6. We've been having sex for a couple of months now and I've noticed that since my sister started having sex with me she seems to be having more kids with less time between their births. We obviously use birth control, but I'm infertile, so there is no chance it's mine.