I saw this post a few days ago and finally had chance to come back today to see what helpful advice had been offered by others.
What a shame that the thread has been hijacked by pages of nonsense bickering.
I can't even remember what Marydoll's comment was that caused someone to get their knickers in such a twist, so I'm sure no one else is particularly bothered about it either. All it has succeeded in doing is adding several unhelpful pages to the thread, which means less people have been back to offer the OP any helpful advice.
On that note, it would be nice if people could get back to offering the OP any helpful advice regarding the situation that she's obviously a little concerned about.
My take on this is that the OP seems to have a good relationship with her DiL, and her grandchildren, and I see no reason why she should feel inferior to the other in-laws just because they are wealthy. It doesn't mean they are better people. Just be yourself, and enjoy the relationship that you already have with them. It may be different to the relationship that the other in-laws have with your DS & DiL, and indeed with your grandchildren. We are all different, and thank goodness we are not all the same!
Just as an aside, my two sets of grandparents were completely different. They came from different backgrounds, and one set were far wealthier than the others. However, as a child growing up, I never noticed any difference between them. They were all just my lovely grandparents who I loved very much, equally, and they loved me. That's what's important. Not how wealthy, posh or well-educated they are.
So, I say to the OP, stop feeling inferior, and just be yourself!