Three's a crowd.

Simon:  Oh, hey--it's the Unbunny! [nose bonks and licks the Unbunny]
River:  Who are you and where did you come from?!

Simon:  It's so good to see you! [lick]
River:  You can lick my head too, ya know! [forces her head between Simon and Unbunny]

Simon: [flops] This is awesome--we have a new friend!
River:  Hmph.  You were totally ignoring me just then.

Simon:  I wasn't ignoring you. [lick]
River:  Well...okay.

River:  You don't like the Unbunny more than me, do you?
Simon:  No, I like you both equally.

River:  But I'm your sister! [gets up and hops away]

Simon:  What did I do?  [sigh] Girls make no sense.
Unbunny:  ...
River was actually jealous of a stuffed bunny.  Poor Simon was in the doghouse for the rest of the evening having no clue what he did wrong.