A new kind of adoption discussion

This was our conversation:

L: So why do you want to keep K?
Me: Huh?
L: (big sigh) Why do you want to keep K?
Me: What you mean "keep"?
L: Don't you want to give her to Texas for someone else?
Me: You want me to give K away?
L: Yes!

I know that kids often want to "give away" their siblings, but how do you explain that you don't want to give a child away in such a way that it doesn't reflect badly on an adopted child's birth parents? After all, L's birth parents did "give her away" from her 4-year-old viewpoint. It sounded very reasonable to her that I could choose to give away her sister.

The conversation ended quickly because L is simply jealous that K's birthday party is tomorrow and her friends are coming to spend the night. (It's just one more perk of homeschooling that we can do things like have slumber parties on Thursday night.) L's own birthday seems so far away!