Oh boy!
"Basically, he said that he had no idea how their treatment of my sister as we were growing up and as adults was something that has caused me so much upset." He'd have to be incredibly stupid/naive to not even have considered it a possibility! Because the difference is quite blatant!
"He said that nothing he or my mum had done was intentional" ugh "we didn't mean it" hardly a robust argument.
"they weren’t aware of how their behaviours/attitudes toward us would cause any ill feeling." Again it's pretty obvious to anyone sensible this is a distinct possibility.
"He said that my sister was always making comments to them about how I’d always been the preferred daughter (in her eyes) and that because I was at Uni it then it must have made them prefer me even more. My dad said they her comments had upset him and my mom and so they had gone out of their way to help her (financially and practically) to show her that she was just as valued as she thought I was." So she emotionally blackmailed and they tried to buy her affections or approval?
The driving lessons issue - again that's clearly unfair and easy to see it is too.
The excuses you're getting are very similar to the ones I get.
"He admitted that maybe he and my mom had been so focused on what my sister needed that they had overlooked that maybe I could have needed some help with things too" this is reminiscent of a conversation I had with my mum after my first breakdown, that I "seemed" to be managing fine - but she never asked, and on the very few occasions she got close to she just accepted me saying "I'm fine" without pressing the issue, which yea or could be argued that I was an adult and could have told her but I was drowning in depression at the time but wasn't aware of it and the way I was raised by the very person saying "I didn't realise but it never occurred to me to ask" was not to make a fuss, not to give her extra stress.
And as you say if you ALREADY feel the less favoured you don't wanna rock the boat anyway.
As for the £200 - helping and supporting your kids when they're kids, even very young adults is one thing, though I think many parents get that wrong too. But when they're proper grown ups with jobs, homes, partners kids etc they need to (aside from obviously if they're in genuine difficulty - nobody wants to see their children or grandchildren struggling or going without necessities) stand on their own two feet! As for "not waiting on payday" that's normal! I'm sure that's pretty much what you do and what they did too!
Equal is not the same as fair! Your sister surely at her age knows that.
"He had said that my sister had never straight out asked them for money at any point" mine never does either - that does actually drive my mum nuts! She'd prefer she were more honest and say "I'm a bit short can you help me out" when actually she does a whole "woe is me" bullshit though more usually she plays it "I hate the kids missing out" cos she knows my parents would never see the kids do without. However what this ACTUALLY means is that money they give her "for the kids" means she's not spending her money on the kids, so she can spend her money on herself (that's the way she justifies it in her mind anyway).
"as in when they went and bought her a new boiler" which they wouldn't have known she needed unless she's discussed it with them PLUS they couldn't have bought the RIGHT new boiler without her input anyway, not how it works.
"I did ask him about the lifts/childcare and what it came down to was the fact that because I’ve got a good set up in terms of income it wouldn’t even occur to him to think of ways to try and help us save money whereas that’s always the focus point of anything related to my sister." Except you actually asked which you wouldn't have unless you really needed that help and then he expected you to pay up for a favour off your dad! As a nurse contrary to what they might think you are not rolling in it - you're just more organised & responsible than her! You shouldn't be penalised for that!
"He said if my sister had a good job and was married (so had a better income) then he’d have asked her for petrol money and lunch too in return for the driving round he did for her, but because she doesn’t then he would never ask." I don't believe that and I was actually quite shocked by that as I don't know ANY parents that would do that, that's tight as fuck!
And his begrudging you 4 hours of childcare when he did loads more for her is outrageously unfair. And again I don't know any parents that would do that even mine wouldn't be that bad.
"my sister does not have a well paid job and isn’t as financially secure" that's her choices and actions have contributed to her being in that position and again not something YOU should be penalised for.
"My dad told me it was kept secret because my mom had said that if I found out then I wouldn’t be happy about it" all that means is that they KNOW they are being unreasonable. My parents do pull this shit, but as I'm now Nc with sis, I have said I have no interest in knowing what they do or don't do for her (there's been a bit of my mum trying to make out she's seen the light and isn't doing as much for my sister now but I know it's not the case and unfortunately other relatives let things slip (even though they know I'm Nc and not interested - I'm VERY guarded about what I tell them as I don't want her knowing certain things either).
They make a big thing of spending EXACTLY the same on me, bro & sis for Christmas and birthdays AND supposedly the grandkids (even though we - bro and I - know and we're not supposed to - that they give her quite a bit of money to give the kids a good Christmas every year as she 'has had it so hard' she hasn't had it any harder than anyone else who made the choices she did!) and apparently the will leaves us all exactly the same but she's had SO much help already it means that of course any inheritance is reduced.
The money isn't the point - what pisses me off is that they CLAIM to act fairly to us all when they absolutely patently don't!
"He doesn’t now, it was when they were much younger. It was about 10-12 years ago that he did it. He provided the childcare for just over 3 years in total." I'll bet her kids are much closer emotionally to your dad than yours. My mum bemoans that she's not close to dd or my bro's 2 - but she hasn't made any effort to spend any real time with them - you reap what you sow.
Acknowledgement doesn't necessarily lead to a resolution which you seem to get and be prepared for.
Also your sister ALSO knows your mothers history and seemingly is playing on that to her advantage.
I'm not Nc with my sister purely because of this. But more because how her being favoured led her to become as an adult. I tried to maintain a relationship but frankly I got sick of her making out she was hard done by while my mum runs herself ragged facilitating her life and barely gets a thank you! Ditto whenever I did her a favour - never a thank you but plenty cheeky fuckery and even complaining if I didn't do a thing EXACTLY how she would even if she'd given no direction. Then there was also outright nastiness on several occasions, last straw moment and that was me done!
My ex was the golden child but then maybe that's partly why he is an ex! He certainly seemed very entitled in many ways, apparently that inc being entitled to have an affair and not suffer the adult consequences (a divorce and an unplanned pregnancy with ow). His 2 brothers don't speak to him & his sister barely does (she's more laid back BUT she also doesn't take any of his crap never has, she's older, he's the eldest boy). My ex in-laws are generally lovely but they did indulge him (he was very poorly when born and almost didn't make it I think is what started it all). He's absolutely fine now! And was as soon as he was sorted at the time!