Trust Robert to come up with such a nice incest story.
Robert's sister, Rose, is totally willing to help her brother collect a semen sample to decide if his testicle, which failed to drop, should be removed surgically, or made to drop.
Being a pair of klutzes, Rose does everything except put the sample in the receptacle. Oh sure, it gets in her face and tits, it's a good thing Rose took off all her clothing just in case he missed, and he did! Three or four times!
In the meantime somehow his cock slips into her cunt, and it was only quick thinking that allowed Rose to capture the essential sample.
And guess what? Who do you think arrives home early?...Yep, Mom! Now I'm a sucker for a good love story, and Robert and Rose live happily ever after, accompanied by their son and daughter, Oh, yes. Is Mom in there somewhere?