Siblings.

River looks so comfortable right now.  Suddenly, an idea forms...

[flop]  There we go!

River:  Can you believe this?  He does this to me all the time!
Simon:  What?!

Simon:  [lick]  See, I'm grooming you!  You like being groomed.
River:  [sigh]

Simon:  Blarg!  You taste kinda gross, though.
River:  What's gross is your tongue sticking out like that.

River:  You're annoying sometimes, but I love you anyway.
Simon:  Good, because you're stuck with me.  Also, I'm not moving--this is comfy!