Here's one that runs about 50 seconds — warm, a little funny, and only lightly climbing-flavored so it doesn't tip into cringe:

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Hi everyone — for those who don't know me, I'm [your name], [sister]'s sibling.

When she told me she'd met someone at the climbing gym, I'll be honest, I gave it about as long as her ukulele phase. But then I met Elliot, and I noticed something. She talks about him the way she talks about climbing — not like a hobby, but like it's just part of who she is now.

Elliot, you make my sister calmer, braver, and somehow still willing to hang off a cliff for fun. That's a rare combination, and clearly you're a rare person.

So please raise your glasses — to [sister] and Elliot. May you always catch each other, on and off the wall.

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Swap in names and you're good. The "catch each other" line lands as the closer — pause a beat before it. If the ukulele bit doesn't fit her, replace it with anything else she was briefly obsessed with (sourdough, rollerblading, a podcast).