"Comparison is the thief of joy".
Silver berry , stop comparing yourself to your DILs family.
My advice is rise above it, there's no need to feel inferior purely because your DILs family are wealthier than you.
Why do you feel that that you "aren't their sort". From what you have said they actually seem rather nice, especially your DIL, she sounds lovely.
I think you need to work on your self esteem and build your confidence.
I was born working class and spent my formative years living in a council house. I am proud of my roots and heritage but I knew from an early age I wanted more out of life. So I changed my stars.
I educated myself, acquired social skills, learned how to "talk proper" 😂🤣, learned etiquette, how to dress appropriately etc.
I can now mix and mingle with ease in any social milieu. Beggars or kings, makes no difference to me.
I've learned that money, class, gender, race, religion are irrelevant. People are just people, treat them all with courtesy and respect and they will respond in kind.
There is always someone richer, cleverer, more accomplished more attractive than me, just as they will always be someone who is or has less of everything than me.
Silver berry. Just be happy in your skin and don't let your social anxiety destroy your relationship with your son, DIL and grandchildren.
You supported and encouraged your son. You gave him wings, you have done a cracking job. Give yourself a pat on the back
and enjoy.