I've been thinking about this and I've been in a similar situation. Like first time meeting MIL I took expensive bouquets of flowers, I was on best behaviour, the best manners ever, offering to help MIL wish the washing up. I don't see what else I could do.
I'm unlucky I guess in the men I went out with were only children, could I seriously expect those women to post on here that they've been hard done to & they've tried so so hard, undoubtedly. Even though it was I who was there thinking what else can I reasonably do.
Do I have examples, oh a library full of them. A family or woman who thought she was something she wasn't, they couldn't be further from middle class if they tried. Spending a fortune on expensive places that scream STATUS, when you're in a shit ton of debt, just completely goes over my head.
Thinking about it, it's something that's bothered me for many years, I do honestly now wonder if MIL sabotaged my bouquet so it would look like shit, plus the buttonholed, because she didn't get her own way. I honestly hand on heart after all these years have just come to wonder if MIL went in and said oh I've been charged with making some adjustments to the flowers, she's a huge fan of weeds, so please can they go with the roses, in her bouquet please can you make sure that it looks like something from a wasteland, because her family are working clash trash, not deserving of our family name.
Please 
because I can laugh too nearly 20 years later, my family member bringing in the flowers looking at me like I must have been on acid when I ordered them. I actually got scissors, everyone was Fairytales no, you're not a florist, let's see what the florist can do. It still came back looking fucking hideous.
We had a wedding planner, who arranged a rehearsal the night before, MIL & FIL refused to go, actually I think FIL came in the end. An amazing guy, loved him to pieces, but my MIL def had poison running through her veins. There's other examples of sabotage, but it might out me.
I recall my first telling off for upsetting MIL, I was bewildered, I asked my family should that have happened. They knew she didn't like us as she wanted a nice middle class girl for her DS. Some of the things she said you'd be 
not just to me, but she did indeed bring it to boiling point where it was make a choice, I was chosen. Which went down like a house on fire as she was pretty sure that without doubt he'd pick mummy dearest.
It's not always Mums, but Sisters have the potential for causing mayhem, creating a situation and milking every ounce of sympathy she could out of it. Yet if she didn't behave the way she did, I don't think it would have escalated that far. She put herself in the situation. She was acting like a crazed lunatic. Again can't give examples of the heights of lunacy as it would be outing. But I know it put MIL & FIL in a very awkward situation. They're nice people they didn't deserve what they had to contend with. But SIL made sure that even when communication lines were down she would do as much damage as possible. I can see the genius of the plan, except it wasn't genius, she put her brother through hell and back by trying to make a point. You could argue you conceded and tried to undo the damage, but by that point there was no magic pen that could tippex out what had happened.
So I always take any posts like this with a pinch of salt. Your version of events she's an unhinged psychopath, her version is likely very different and can highlight exactly what you did and when. I can't buy for one minute that you're not peddling to DS oh why don't we do more with DIL if youre truly in the dark and unaware. Did any of your siblings bully DIL in the past?
As a possible scenario that no one has done anything wrong, there's the chance she feels you just don't have anything in common. That's that. It's up to your son to facilitate you seeing Grandchildren.
I think really if DIL ignored you the first time you said hello, going out your way two further times to say hello was to make her look like a twat in front of her boss. You were the visitor so you had superiority, there was no way you were going to let that pass. You had the upper hand and used it to your full potential. You think her boss didn't bollock her for reducing poor MIL to tears. You don't think that further added to your list of problems?
You've got enough people saying oh DIL & her family are awful. Someone's got to shine the torch for your DIL & play Devils advocate.