Bathing my labs

Question
I have two labs, one yellow (Nalle) who is 3 years old and one black (Oso) who is 4. Nalle is CRAZY about water, Oso not so much. Every time I give them baths I take Nalle first and then Oso. But when I'm bathing Oso, Nalle starts barking at him, although not in an aggressive way. It's just funny. And then, when Oso is clean and "dry" and I let him go, Nalle goes after him and starts biting (more like nibbling) his neck in a playful way. She doesn't hurt him or is aggressive towards him, still I'm curios to know why she does that. Her behavior just makes me laugh.

Answer
When it comes to behavior, much of the why is guesswork.  It could be jealousy.  While you are bathing him, you and he are interacting leaving her out.  She may be more interested his attention than yours.  That reminds me of years ago, our neighbors and us both had a dog.  When I would mow along side the fence separating the yards, their Collie would run up and down the fence barking at me.  Our little mix would go berserk running up and down the fence where I was trying to mow.  He was upset at her paying attention to me.