Why is he DOING this???

Doom has, oddly, taken to peeing on Bobsie's head the last few days... She will be outside so they can do their business, Bobsie will be sniffing for THAT SPOT, and Doom will come over and hike his leg right over her. She's had five baths in the last three days.
What the heck is going on???

ETA:
(MATURITY WARNING)
YouTube - [Dave Chapelle] Piss On You
This is Doom's new theme song, according to my friend Kate.

He wants to mark her so the world knows shes his LOL.

Idk Sci used to do the same thing to Walker.

Should I just leave it one time and see if he does it the next time we go out??? That just seems so gross!

LOL I dunno we didnt do anything about it but to walk walker on the other side of the street.

omg, my dogs do this to each other, but by accident. It goes like this:

-dog 1: sniff, sniff, sniff, what a fascinating spot this is.
-dog 2: ooh, what are you smelling? let me smell it too!
-dog 1: yes, I think this is a perfect spot for a pee.
-dog 2: sniff, sniff -- hey!

I've experienced that once or twice with my dogs. I would simply go out with them when they pee every time for a while and interrupt him (so he can't practice this behavior) with a communicative/mild "eh-eh" and herd him away from her. Then praise/treat when he goes in his own spot which is away from her. If he wants to pee where she peed after she leaves, fine. But yeah, it's not fun to have to give baths all the time and he can learn that it's not nice to stink up his sister. lol. And that it works better (to get treats) to pee elsewhere...not on her. Marking the spot where she peed will have to suffice.

Doberluv has a good idea...but also, if his sniffing Bobsie's head a lot first, you might want to wash her with some vinegar or something. But then again, if he's only doing it outside, that may not be the problem...could be worth a shot, though.