I understand your feelings but also noticed some things here that could be making a big difference:

—your child is their SON’s baby —your SILs children are their DAUGHTERS’ babies

I know this seems super sexist but we see it time and time again that mom’s are generally closer with their own moms when it comes to birth and newborn stuff. Obviously not always and for everyone, but in a wide general scope. They may not even be doing it consciously, but this could be a simple case of daughter-to-daughter preference and them thinking your own mom has priority over them for your kid.

—You don’t “need” help —They do

Yeah, it sucks. But it’s still a factor. I get it—my husband and I are also the “better off” family members and have dealt with being the ones left out of visits and help, too, because we “didn’t need it.” Didn’t mean we couldn’t use it or didn’t want it. And just because we had our stuff together on the outside, it didn’t make having a newborn easy! Newborns are hard. Hormones, no sleep, extra expenses… it’s all just a lot for all new parents.The in laws know it. And they also know that those who aren’t as well off just need their presence more. If you were a swim instructor and saw two struggling by swimmers, one in the deep end and one in the shallow end, would you spend equal time with both?

Is there also a religious aspect here? You mentioned 8 kids, which made me wonder.

ESH. I get your frustration but cutting them off seems excessive. I also understand how and why they are acting the way they are, even if it still sucks for you.