How do I back out of a rushed concert-ticket deal without letting my sister down?

The seller sent a screenshot of two seats and wrote, 'Someone else wants them. Send the $260 in ten minutes and they're yours.' I had only asked whether the tickets were still available.

This is supposed to be my sister's birthday outing. We missed the original sale, and she found this person in a fan group. The profile has years of band photos. The seller says a family problem means they can't go, and wants payment through a money-transfer app before sending the tickets.

My sister offered to pay for one seat. 'We finally found someone charging what we can afford,' she said. She thinks my hesitation means I'm quietly abandoning the present. I want to go too, but I haven't independently checked the seller or how tickets for this event can be transferred. No money has changed hands.

I'd rather lose this offer than rush a payment I can't assess. How do I tell her that while still taking responsibility for planning her birthday? A different night out won't feel like the band she's waited two years to see.