She's allowed to see her mother as much as she wants. Why on earth should she have to not see her (even everyday if she wishes) because you might not like it.
If you're not getting to see the baby enough, why not take it up with your son? He's the parent as well, why should all the blame go to your DIL? (And I say this as someone who gets on well with their MIL, so no MIL hating here).
Also, threatening to refuse to see your GS is ridiculous. You'll be the only one losing out there. People complain all the time when DILs use GC as weapons against GP, by threatening to not spend time with him because the other nan sees him more you would be no better (or worse really, as you must see him often enough anyway as you say you take him swimming!)