Agreed, Madgran and Smileless. Not to mention that AC/DILs have often come onto the support threads for EGPs. This is a public forum (as they generally point out when confronted) and people can read and post wherever they choose. I think the difference is in whether they are critical (and sometimes cruel) or supportive.
Starblaze, I'm glad you have a good GP role model in your dad. And I applaud your older children for distancing a GM who played favorites and put down their mum (you). I think it's harmful to children to constantly be exposed to someone who badmouths one of their parents. In some cases, it can be damaging to the parent/child relationship (fortunately, not the case w/ your children). This is important, IMO, b/c it can affect the degree to which the kids listen to their parents about drugs, drinking, etc. Unless the parent, themselves, is abusive, the parent/child bond needs to be firm. And so I feel GPs who consistently undermine that are doing harm, whether they mean to or not. (NOT saying any GP here ever did that.) Again, luckily, Starblaze, that didn't happen to you and your kids.