Norah, I get that your sils made the decision to co their mums and that your dds are only co from them AS A Result of this. However, from what Iv seen, dils CAN decide to co mil if only from themselves. IOWs, dil can decide, "I'm not speaking to or spending time with mil anymore.' Her dh can still visit with his mum and maybe even bring the gc, if any, to see her, but dil can go totally nc.
In one or two cases here, Iv seen where ds visits his mum, but dil AND the gc are co. Maybe ds and dil made the decision together to keep the kids away from mil, but who knows?
You're right, of course, that only ds/dd can co their own mum from their whole nuclear family, including themselves. Dil/sil might influence that choice, but, ultimately, only ds/dd can make that decision (different if they're victims of spousal abuse, of course).