I wouldn't worry about this OP. I like my partners parents but it wasn't until my DD was born that I really spent time with them. Prior to that I would just attend birthday/Christmas/
Mother's Day events. But equally DP would only have that level of contact with my family. We like each other's families enough, but we just always saw our respective families as our own 'duty' to visit until DD was born. Since then we have spent more time with each other's families, but DP still rarely visits mine, and I visit his as he asks me to. As long as she's not stopping your DS visiting you then I wouldn't worry about it.
It may be that they don't have children for a very long time and you could be stressing yourself for years over nothing. If your son still visits and she's polite enough to you when you see her then I would see it as a blessing, at least you don't have an evil DIL
I would just keep the invites going so that you've kept the opportunity for a relationship open, but expect her not to come. Then if they do ever have children at least it obvious that you've tried to build a relationship with her before. Like I said, I never made a real attempt with DPs Mom, just as she didn't with me. But when we told her I was pregnant she was suddenly all over me like a rash and that was quite irritating as she hadn't wanted a relationship with me before either, so it was quite obvious it was only due to me carrying her grandchild. So I would keep the offers going, at least occasionally so it's obvious you would like a relationship with her as a person, rather than just due to a pregnancy.
I wonder though, how often do you see her (you say DIL, so I assume they're married) and you must have at least seen her at the wedding? And how often are you asking to see her (and your son)?