Stuck on what to write in a congrats marriage card (delicate situation)

by J
(PA, us)

I'm so stuck. I am normally very good at coming up with a nice & personable message in cards, but because of the Dynamics of this situation, nothing feels right.

Here is the tl;dr:

My SIL died 5 yrs ago and her widowed husband has remarried to someone we really like, but grief is still ever-present & I'm having a kind block as to what to write in their congrats card.

Here are the specifics....

My sister-in-law (my husband's sister) lost her battle with cancer 5 years ago. She was 31, and was survived by her husband, son (now age 11), and daughter (now aged 7), and was probably the sweetest & purest soul I've ever known. Her husband has been very good about keeping ties with our family, and my 2 daughters are great friends with their cousins. He recently got married to someone he has been dating for about a year or so, and we're happy that she is officially part of our family. She has been very careful about not trying to be the "mom replacement". She's done a lot to keep my SIL's memory alive, such as putting photos back up on the walls, as well as taking on the crafts & projects that my SIL always did with the kids.

Their wedding was very low key, but we (my husband & kids, and his parents) did not attend (technically, we weren't invited..). When they first became engaged, they were planning a larger wedding, and she was very respectful & considerate of my husband's family's feelings. She verbally gave us an informal I before anything was planned, but understood if we felt it too difficult to go. I had kind of figured on going with my daughters, as to represent our family (and because the cousins are so close), but then they abruptly got married by a JOTP & had a very informal get together that we were never told about (probably to spare our feelings).

But every time I try to come up with something to write, I keep drawing a blank. I'd like to officially welcome her to our family, but I'd also like to acknowledge that he has found happiness again. I also don't want to leave out the kids (especially the son, who to this day has a difficult time coping with the loss of his mom). I don't want to diminish the fact that this is her first marriage, and I also don't want to ignore the elephant in the room (my SIL's memory). We've already had talks with her & told her that we're so happy they found each other. We've also expressed that we know SIL would approve & probably would've loved her too. I should probably also acknowledge (or apologize) that it's taken us too long to get a card out to them.

How do I say so much without over-complicating or over-formalizing it? This card is so late going out as it is- I can't sit on it any longer.

Any ideas, inspiration, and/or advice would be very greatly appreciated!!