You are taking your first steps towards asserting and managing your boundaries.
Look on this as practice, not a failure.
It will take a while to get it just right, some people have had a lifetime to practice, so of course it's going to be harder for you, playing catch up.
In a very nice way, I do think your last few posts sound a bit of a 'cutting off your nose to spite your face' kind of reaction. The way you're saying 'fuck it, if I didn't get it exactly right I won't ever bother again'. I get the frustration, and hope you're just venting! Venting is good!
But if you are genuinely going to give up and allow people to trample all over you, the only person you'll be hurting is yourself. Please don't do that to yourself!
Keep going and don't give up, so you learn and you get better at it. It will be worth it for your own happiness and wellbeing in the long run.
sleep had some good practical advice when s/he wrote:
"You have every right to set your own boundaries. You have no right whatsoever to seek to set other people's"
That's what you need to take out of this situation. Not that you fucked it up, just that you got it a bit wrong and you can learn from why. You got suckered into the 'poor martyr' grandma who has her own reasons for not being completely coherent in the way she portrays her reality to people.
You slipped into ignoring her own adult-ness and responsibility for her own decisions and boundaries. You thought she needed someone to step in on her behalf, and that she couldn't be held responsible for anything that happened that day. When in fact, she had her part to play in letting that happen in the way it did. Not pointing fingers or apportioning blame, just recognising that she had her part to play just like the two sisters did, and the brother did too, and of course you did, but to a lesser extent as you're not usually part of their family dynamic. They brought in their daily dynamic and issues and that's why the situations happened. And you saw that it's not a good dynamic for anyone, especially the children, and you reacted by identifying the 'aggressors' and the 'victims' and assuming it was a clear cut situation where you intervened on behalf of the mother/grandma who you had pegged as a victim in this who wanted to be saved, and who also would agree with your summing up of how bad the situation is for the children.
So now you know that you can't discount another adults role in a complicated situation.
You can't step over the grandmas boundaries in order to push back on your own behalf (and hers by default). You can't do that even if it looks like the right thing to do, because you end up over reaching and setting yourself up for a fall.
You have to keep focused on protecting your boundaries first and foremost, although that may seem an uncomfortable thing to do (I'm rubbish at it to be honest).
All you can do is identify and maintain your own boundaries.
It's really tricky I know! I'm on that same journey of learning how to recognise and uphold my own boundaries. It's bloody hard! It's really difficult to train yourself to think in a totally new way and also then how to act on this new way of thinking in all sorts of different situations thrown at you. Especially when you have to unlearn other ways of being to make way for new healthy ways to see yourself and your boundaries.
Think of yourself as having been given a surprise practical test in your first year of your A-level in Boundaries and self esteem 
. Silly I know but bear with me please, if you can stop cringing long enough there is a point to this analogy, honest guv!
So, well done in coping with this surprise test your tutor has rather meanly sprung on you and your class!
For extra credit you had to identify the tricky double bluff bit which came in the form of grandma and the sisters existing family dynamics and the grandmas representation of self to different audiences! Ha, the devious mind of the examiner, putting that in to trip you up!
You got full marks for the foundation level of 'identifying your own boundaries' (yay! Big ticks and many marks given, it's harder than it sounds!), and then you completed that level well by a good effort at the 'maintaining boundaries and defending them' level... a good attempt but dropped a mark in the way you defended them, but you still got a good pass as the main thing is you did try and defend your boundaries in some way. (Yay, more marks and a definite pass for you, woo hoo, and perhaps a hi five to self!).
Unfortunately on this occasion, you didn't get the final advanced level points for 'tricky family dynamics and the introduction of a victim presentation of self'.
Never mind though, you got a good solid B- 
. It was a toughie and of course, you are only in the first year of the course, and you didn't do the gcse level in this subject did you?!
Next term you'll get onto the final and trickiest topics of the course... the changing dynamics of dh's, and their not so good behaviour either rooted in their own historic family role playing, or their resistance of changing roles in the partnership for their own rather unpleasant motives.
Changing yourself, changes the dynamics of all of your relationships, and people often don't react well to change, even if it's for the better. But that's for another time... another lesson, another term!
So, big smiles all around and congratulations, before you break up for the summer hols.
Right, having flogged that analogy to death I think I'll leave the poor thing flailing on the floor!
Please forgive me for the flurry of imagination there, but in my somewhat heavy handed way I was trying to make the point that you don't expect to be able to master skills first time in an area of life/learning where you have such a big gap in your learning! And you'll probably be a learner for a while yet, if this is your first attempts at doing this tricky boundary stuff.
And that's ok.
Don't feel a failure or give it up as a lost cause. So, you didn't do it perfectly.
But it's bloody hard and the fact that you tried at all is a massive achievement (if you're anything like me & countless other people who are learning this stuff later on in life than it should have happened). You wouldn't tell your 6yr off for not knowing their 3 times table would you? You'd know that they'll get taught that again and again all the way through yr2, and if she masters even 2x3 = 6 in yr 1, that's great and more than she has to know this term. You'd say well done for trying and I feel it felt right, you might bring up counting in threes again if you happen to see things that might help her as you walk past... you would never tell her off for having a go and getting it a bit wrong would you?
Or if the whole school exams analogy isn't working for you, what about thinking of it like learning to drive? You can only go so far with theory learning. At some point you have to get behind the wheel and actually drive the bloody thing. A huge leap of faith (& trust in your driving instructor). But you cannot learn to drive unless you keep on getting back behind the wheel and having another go, and another, and another. After loads of practise and mistakes and a few near misses, it all becomes second nature, automatic and you're not even aware of thinking about it.
You're at the stressful leap of faith bit where you start driving the car, and you will only get better if you keep on going, keep getting back behind the wheel.
Don't give up, and maybe try and be a wee but proud of yourself for trying. You didn't crash, the car's not a write off and no pedestrians have been run over, so it counts as a good lesson!
I will say that your later posts hint at some underlying tricky relationship dynamics which are probably more of an important issue than these bloody awkward relatives... if you can bear it, I'd recommend turning your focus to your dh and thinking about the dynamic there i.e. There's probably a reason he's not being supportive towards you, his partner, beyond the tricky extended family dynamics of this day.
Does he have a vested interest in you not learning how to manage your boundaries and start letting yourself have needs etc? Even if he's not aware of it, he didn't support you as well as he might have, in fact he just left you to flounder.
Maybe you could start a thread in Relationships exploring that side of things, and getting some support for your learning about boundaries. There are some very wise mumsnetters about all this boundary stuff and I think that type of thread might be more helpful to you than this err, feisty debate (?!).