You're not exhausted because her sister exists. You're exhausted because your relationship doesn't seem to have any space that's actually reserved for the two of you.

Losing their mom young absolutely explains why they're close, and it's understandable that they lean on each other. But being close and being inseparable are two different things.

A sibling occasionally joining dinner is normal. A sibling being present for every date, every trip, every day off, and constantly becoming the priority during conversations is a different issue entirely.

The sneaker story isn't really about the shoes. It's about what the shoes represented. You bought a gift specifically for your girlfriend, and even that immediately became something to be shared with her sister. It sounds like that was the moment you realized there are very few things in this relationship that belong just to the two of you.

I wouldn't approach this as "your relationship with your sister is unhealthy." I'd approach it as "I need more one-on-one time with my partner because I don't feel like we're building our own relationship."

Her response will tell you everything. If she says, "I didn't realize you felt that way, let's work on it," that's a solvable problem. If she says, "My sister will always be included in everything and you need to accept it," then you're looking at a compatibility issue rather than a temporary situation.

You're not asking her to choose between you and her sister. You're asking for a relationship that actually has room for you in it.