Hi,
If I were you, I'd start with a conversation -- perhaps a scheduled meeting, even -- with her teacher. Tell her that you know she's doing a great job with your daughter, but that you wanted to let her know of your concerns based on what your daughter is expressing at home. This is information she has no way of knowing and *should* be thankful to hear. Also let her teacher know that, while she isn't bored at school necessarily, that she does already know most of what's being taught. Say something about how you know how difficult it is for teachers to teach to the whole range of abilities in the classroom. You could even play the humble parent card wherein you say something like, "I know all parents think their kids are smart, and I'm not saying mine is a prodigy, but in light of her brother's scores/our family history/what we've seen of her abilities, I think that it would be a good idea to assess our DD so that we can get an idea of what she needs." Tell the teacher that you're considering whether the situation can be left alone, whether you should try to work out some individual instruction, or whether you should consider an acceleration of some type. But tell her that you feel like you need some cold, hard data to help you make that decision. See what the teacher says, see if the teacher is willing to talk to the principal if you two agree about what is needed, and if that doesn't work then see if you can talk with the principal directly.
Since they have been sensitive about you approaching them with your son and have claimed that their teachers know what their students need, going through the teacher -- who knows her the best of anyone at school -- might be the most tactful way to start.
BTW: When my DD was being evaluated by the school and scored really high, they casually mentioned that her younger brother was probably pretty smart too (he hadn't started school yet), so I thought that it would be obvious that he might get a closer look without my having to say anything. But, the school has a lot going on and it just slipped under their radar. To their credit, though, when we approached them about my DS skipping K, they didn't hesitate to test because of our track record with them of being reasonable, concerned parents who turned out to be right about our DD's abilities.
HTH!
