Male cat urinating in his water dish

OK this is the most bizarre thing I've seen.  I live with my sister and we have 3 cats (2 are mine, 1 is hers).  Their food/water areas are separate.  I have the one for my cats in my room and she has the one for her cat in her room.  Within the past couple of months I've noticed her cat has been drinking out of the water bowl my cats use .... a lot.  Not to mention he's constantly scratching at the floor to mark.  To complicate the matter I have found piss in my sister's cat's water dish three times now.  

All the cats use the litter boxes to urinate/poop.  There's one in my room and one down stairs in a spare bathroom, but they predominantly use the one in my room.

What could be the issue here?

EDIT:  Nevermind guys.  I finally brought it up to my sister and she just texted me back saying it's some drops she puts in his water to aid digestion.  

I guess that makes sense ..... I mean the yellow discoloration didn't smell like piss.  False alarm.

My guess would be that he is drinking out of your cats water bowl because he doesn't like the stuff that she is putting in his water. 

Scratching on the floor is not marking.  My cat "buries" what is left in his dish when he is done with his wet food. He scratches on the floor around the dish but does not leave marks.

As for using the litter box in your room.  Maybe because it's closer and they don't want to go downstairs or they like the litter better than what is in their box if it is different.