Certainly anyone who is taking that big step of commitment needs to be congratulated!
Whether it is a grandchild who is the actual direct participant, or that person themselves, the event will, we hope be a joyous occasion for all, and with all the concommitant features now, and in the future: a long-time happy relationship; off-spring to delight and confound (in their teenage years) the whole family, and the unavoidable promise that life goes on.
Ah, promises, promises, promises. What is that well-known Jewish toast?
Ah yes, l'chaim!
And then there is the Slavic: Na Zdorovie (На здоровье), which my Ukrainian wife prefers.
In whatever language or dialect you do it, there can never be enough of them!