While not as wealthy as your DiL's family are Silverberry, I have been on the better off side of 2 marriages. My parents were better off than DH's, my father was an army officer, DH's father worked on a car assmbly line, and we are better off than DDiL's, whose father died when she was 5 and whose mother has not remarried.
i do not think in either case, we have ever looked down on or compared ourselves with our fellow in-laws, nor flashed our better off life. In each case everyone has got on really well. and differences of income and life style have never come between us.
My DDiL, like us, is from a small family and the two families have become as one. my associate grandmother and I ring each other up regularly for a chat. My MiL, after she was widowed, spent holiday's staying with my parents. Money never came into it. As individuals there was more that united them than money could ever divide.
Yes, we can now and again provide family treats that would be beyond the capacity of DDiL's mother: holidays, help with cars. but we do not look down on her because she has less, and she lives closer than we do and has been able to provide the physical day to day help we could not.
When our DC stayed with their paternal grandparents, they travelled everywhere by bus, as their grandparents did not own a car, and experienced a good old fashioned small town life, walking to the shops, stopping to chat to all the neighbours, who would remember their father as a child. they loved it - never thought of their paternal grandparents as being 'poor' in any sense.
As for the incident with the stool. To be honest I would never risk buying anyone either clothes or home furnishings, whether they were less well off than me, better off than me or whether our incomes matched to a penny unless they had told me exactly what they wanted. Taste in clothes and house furnishings can be very different and one person's lovely sweater( stool), just right for the recipient, can actually be a colour they never wear or in a style they loathe. Money has nothing to do with it.
You have a lovely DiL, grandchildren who love you, what more can you want? Do not spoil a lovely relationship with your son's family with silly old fashioned notions, nor blame incidents like that with the footstool on money. It had nothing to do with money and everything to do with taste and that applies to everybody.