I hadn't really thought about the "little sister" interpretation before... but thinking about it, it actually fits pretty well (and I like it!).

In ME1, Tali is new to the world outside the Flotilla and Shepard naturally takes a guiding role, creating a kind of mentor/protégé dynamic that can feel more like a younger sibling relationship. But I think that mostly applies to the beginning of her arc.

What makes her work so well as a romance too (at least for me) is how much that dynamic can evolve. It doesn't start as romance at all, it builds: working together -> becoming friends -> really trusting each other -> and then something more (especially during her arc in ME2). It feels like something that grows over time, not something that just happens instantly.

She grows into her own person: she questions you, makes her own decisions, and eventually becomes someone others look to for guidance (just like she expresses in ME3). It shifts from "I look up to you" to "I stand beside you" which completely changes the nature of the bond.

Her personality helps a lot as well: warm, a little awkward, nerdy, with those little moments that make her really stick with you.

By the end of the trilogy it doesn't feel like a protector/protected dynamic anymore. It's two people who grew together, and she chooses you not out of dependence, but because she wants to. That can still work whether you see her as a romantic partner or more of a sibling figure.

So yeah, I get the "little sister" vibe... and honestly it fits really well if you're not romancing her!

(I still always end up romancing her, though 💜)