Here's a solid under-a-minute one with no cringe. Just swap in the names:

**Main option - warm & simple [~45 seconds]:**

> For those who don't know me, I'm [Your Name], [Sister's Name]'s [brother/sister].

> I've always known [Sister's Name] as someone who goes all-in on what she loves - whether that's a tough climb or the people in her life.

> So it makes perfect sense she met Elliot at the climbing gym. He keeps up with her, challenges her, and makes her laugh while doing it.

> Watching them together, it's clear they're a great team on and off the wall.

> To [Sister's Name] and Elliot - wishing you a lifetime of adventure, laughter and looking out for each other. Cheers!

---

Two other vibes depending what you want:

**2. A bit lighter / funny:**
> Hi everyone, I'm [Your Name], [Sister's Name]'s [brother/sister]. For anyone who's been to the climbing gym with [Sister's Name], you'll know she doesn't do anything halfway. So when she wouldn't stop talking about a guy called Elliot from climbing, we knew it was serious. Elliot, thanks for matching her energy and for making her so happy - you two just work. Welcome to the family. To [Sister's Name] and Elliot!

**3. If you're nervous and want 30 seconds max:**
> I'm [Your Name], [Sister's Name]'s [brother/sister]. It's been so lovely watching [Sister's Name] and Elliot together since they met at the climbing gym. They're kind, fun, and make a brilliant team. Elliot - welcome to the family. To [Sister's Name] and Elliot!

Tip: Say it slowly and pause before the final "To..." - it'll land better and still be well under a minute.

Want me to tailor it with your sister's name and if you're her brother/sister?