I saw this, too.  She is thinking, "They might say no to me but they won't deny their little favorite anything".  Boo boo.  So annoying.

So this is going to be super petty, passive aggressive advice (lol)

That would annoy the hell out of me, too. I'd walk out and say "SD, that's cute you have a 5 yo come ask/speak for you, it's like she's the big sister, huh." 

My SD is 12 too but she uses her 27 year old aunt to do this. 12 year old girls are a combination of insecure and entitled and they constantly try to test relationships and to see what they can manipulate people into doing. If you don't shut it down it does get worse and worse as they get more sophisticated and smarter at it.

I can relate. My SS does the same thing, although it's decreasing. I have been teaching him to ask directly for things. I let him know that just because he asks doesn't mean he will get, but he needs to ask. He's starting to be more direct, and using my bio less for this sort of thing. I mentioned this annoying behavior to mine and my H's marital counselor, and counselor says it indicates SS is used to not getting his needs met, which reflects his BM and H's parenting over the years. I think you have done well by raising your BD to have a voice. I also agree with you in telling her to say no to SD, and to tell her to go ask for herself. If this is a pattern sort of thing, then that's definitely appropriate, as it is basically manipulative behavior if a pattern. Kids need to be taught how to stand up for themselves against this kind of stuff, otherwise we are stuck dealing with it still as adults and being somewhat oblivious to the manipulation or stuck not knowing how to deal with it and just getting walked on. I would step in the next time it happens, though, if you are aware of it. Your bio might need support in taking a stand, and I also think it warrants parenting on yours or your H's part towards SD. Tell your SD you are aware that she is asking sis to ask for her, and that she needs to start asking for herself. Ask her what do you need? And tell her you need to ask me or Dad directly. If she continues the behavior, I'd go so far as to decline whatever the ask is in the future. Like, you used sis again to ask, and you have been told not to do that, so the answer is no this time (if it's not a basic need, like food, etc.). Tell her "Ask for yourself next time, and maybe you'll get a different answer." Best of luck to you! This stuff is so annoying. But it is rewarding for everyone when there are positive results and things change. Things have changed quite a bit with my SS on this over time, so I think there is hope.