Hi there, sorry that you're going through this, it sounds very stressful. I haven't seen the other posts so I don't know all the specifics but I'm sorry for your loss.
What's the reason that you don't want your children to see their grandparents? It might be important to them, especially as they get older since their father had passed and their grandparents may be the closest thing to him.
If they do genuinely believe how you treated their son contributed to their child committing suicide (even if this not the case at all). You could understand their upset although it's not appropriate to write negative things publicly, Grief can do awful things to you and it's a shame that they didn't show more restraint, but again we all don't know how we would act if our child had died and we thought someone's actions contributed directly to our child ending their life.
Personally if it was me, I would do mediation with the family and explain that they need to be more respectful towards you and permanently stop any negative comments and you can build a relationship and they can gradually build up seeing the grandchildren as you don't trust them at the moment to not say negative things about you to them. If they respect this then they see the grandchildren but if they can't do this, then it won't work out.
Then main point in what I'm saying is: sadly children have lost their father (not undermining you losing your fiancé, I'm sure you're still heartbroken too and miss him and it must be hard being a single parent after that). But children do come first, and they have lost their father. It might be important to them for them to be able to establish a relationship with their fathers family. I know if I were to have lost one of my parents then I would want this. Or if my DP passed away, I would let my children see his family (most of his family I don't even like! But I still would, for the children).
But to answer your main question, any friends that have entered family court are normally favoured if they're the main resident/full custody parent. It's not common for other family members to be given legal rights against the residence parents wishes that I've heard of.
But it should be a last resort battling in court and letting an outsider make the judgment weather your children can see their deceased fathers family or not. Mediation would be my suggestion, for the children's sake.