I'm not a little but felt like I had some input here. I apologize if it's truly inappropriate of me to be answering you in this section.
Older Brothers/Big Brothers and Older Sisters/Big Sisters exist as valid identities, but I feel like they are more of a rarer identity right now. Generally, the Older/Big Sibling role does involve a level of being regressed (or in littlespace as people say now) but still being capable of taking care of another person who is regressed too at the time. So, they may perform diaper changes or set rules/structure but are also possibly interactive in playing the same games and performing the same activities as their little sibling. They may or may not be as young feeling as their little sibling but are certainly more comfortable with the idea of being the person "in charge".
Maybe that's more your identity though. A little that is "in charge" but not necessarily in a bratty way but more in a loving, caring way.
Lately, the only time I see the brother/sister roles presented has been in a non-serious manner from littles who are wanting extended-but-not-intimate family-style dynamics or want a third-wheel to get sexual gratification from sometimes. Most of the time though I see it's like having a non-sexual poly-style family dynamic that is usually not very seriously devoted to one another or a sexual poly-style family dynamic but only for specific scenes.
I believe this detracts from those Big/Older roles from actually being valid though because they can truly be entire relationships in themselves, can be romantic, and can be sexual if the big/little persons desire that and don't have to be fun extensions to play with when one is bored or aroused. Relationships that involve a Big/Older sibling don't need to have a Mommy or Daddy to be the caregiver or the person-of-authority in a relationship to still have a healthy CG/L relationship. A little can provide care while still being and feeling little.
As you can gather from what I've said up to this point, I strongly feel that CG/L isn't just Mommy/little or Daddy/little but can be a multitude of identities, sometimes vague enough to just be Caregiver/little. Caregivers can be Mommies, Daddies, Aunties, Uncles, Big Brothers, Older Sisters, and onward just as a little role can be adult babies, gender-neutral littles, littles, middles, teens, and so on. These are all valid identities and none of them are strictly dominant and strictly submissive in nature.
Also, just as an additional subnote, I'm a Mommy but don't identify as dominant at all. I strongly feel that the dominant/submissive aspects are subjective to the relationship at hand and are not necessarily the standard BDSM-definition of CG/L relationships. I think I explained my stance on feeling submissive but still as a caregiver personality and identity here http://www.littlespaceonline.com/viewto ... 907#p35907
That being said, I've interacted with a couple of dominant-feeling littles previously. Majority of those littles identify as brats who don't submit to punishment and who want a twist of a parent/child relationship where the parent is taking care of them while feeling submissive. I've met a couple of caregiver-identifiers (usually Daddies) who are seeking a dominant little (usually a brat) because they like the blackmail or findomme/findom adult interests that can come along with it even. It's the idea of being wrapped around someone's finger, perhaps made to take care of someone, or being so incredibly submissive that even a younger mind easily overrules you. I can somewhat relate to this desire for "taking care of" another person in full while feeling in servitude but it's less sensually-driven in motive for me than I've encountered with most.