Another belated happy birthday beautybumble. There is absolutely no excuse (barring emergencies) for your son's behaviour. Whilst it's true that younger people cannot possibly relate to being old and alone, they know perfectly well what a birthday is, especially when it's their own. I'll bet they don't forget those.
It's the one day of the year when you can show you are thinking of someone and, in your case, acknowledge all the help your son and DIL receive from you.
I feel angry on your behalf and personally if it was me I'd jolly well show it. They deserve to feel guilty.
Have you ever been to see a Spiritualist



Also D1 and SIL’s birthdays are 2 and 3 days before mine so of course their “big” birthdays have in the past taken precedence.