Here's one that's warm without being saccharine:

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Hi everyone — I'm [Name], the sister, and I've been told I have one minute, so here goes.

Some people meet on dating apps. Some meet at parties. My sister met Elliot because he was apparently impressive enough on a climbing wall to catch her attention — which, knowing my sister, means he's either an incredible climber or very good at falling gracefully.

What I've noticed since they got together is this: they belay each other. Literally and otherwise. One of them is always steady while the other goes for it. That's kind of the whole thing, isn't it?

[Name], you've always been brave — on walls, in life. Elliot, thank you for being someone worth trusting with the rope.

So please raise your glasses: to [sister] and Elliot — may you always catch each other.

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(~35 seconds at a natural pace.) Swap in actual names, and if there's a funny real detail — like who fell first, or a specific climb — drop it into that second line. Specifics kill cringe better than sentiment does.