Who gets the evening slot when our Steam Family has only one copy?

My sister knocked on my door at 8:20 and said, "I bought it. Can you stop now?" I'd been playing the shared game for an hour and was in the middle of an encounter I couldn't finish quickly. She'd just come home and wanted to use the only copy in our Steam Family.

We're adults living in the same household. I had contributed a different game to the family library; she had bought this one. I knew sharing didn't mean we could both use this single copy at once, but I hadn't asked about an evening slot because she'd worked late all week. [Steam's current family FAQ says that when no other copy is available, a family member can wait or buy another copy](https://help.steampowered.com/en/faqs/view/054C-3167-DD7F-49D4). Neither option tells us whose Wednesday takes priority.

I asked for twenty minutes. She said that if she has to book time for her own purchase, sharing is costing her rather than helping. I said she could use the game I added while she waited, which came out more smugly than I intended.

I can avoid her usual hours, but her shifts change. Would an owner-first agreement with a short finish-up window be reasonable, or should I stop relying on access to this game unless I've asked that day?