If you told me your son had got engaged, I'd probably say something about that's exciting and ask when they were having the wedding.
You said they've known each other for years. Are they living together? When someone who has been living together gets married, I'm usually a bit meh. They're already sort of married in practice and this is just a formalisation really.
I wouldn't contact a relative I wasn't close to about an engagement if they didn't tell me personally. If they don't share it with me themselves, I'm not that involved and more of an outsider. If I'm close, I might send a card.
If they posted on social media, I'd probably just post a brief, congrats. Maybe ask when the wedding is.
The variables for me are how close I am, whether I'm involved or on the outside looking in as a more distant family member, whether they are already living together.
When my child got engaged I'm not sure any of the relatives, including my sibling, sent a card or did anything much. They weren't living together till marriage and the wedding was where we all celebrated. No-one felt anyone had done anything inappropriate.