Sister relationship + ADHD reactivity

So today I lost my temper at my sister and I need advice on how to approach a resolution, as RSD and ADHD make it hard for me.

Backstory, she and her kids are living with us due to relationship breakdown, and there is a lot of things that are annoying about that which I just grin and bear it - that’s life!

but today, we had agreed and confirmed she would have my child to take care of her while I work from home (stressful white collar job that often sees me in 6-7 hours of meetings and you are not allowed to be child caring at the same time). So when I went downstairs she announced she was leaving soon and might leave my child here for an hour or two while she took her daughter to swimming - if it was ok with my daughter (6yo). I answered that that would be ok today because I didn’t have morning meetings but if my workday was similar to my schedule the day prior that actually wouldn’t work (I.e. not really up to her or my daughter based on what we agreed) . She didn’t really say anything just looked at me. I then rushed to get my younger child ready so I could drop her off and be back (presuming she needed my car with the extra seat in order to take her two and my eldest).
When I got in the car she asked if I was leaving, I said yes to drop my youngest off, and then I realised she was going to leave, and I said, you’re not leaving my kid here unattended right? She said aren’t I just going 2 minutes down the road? And then ‘I told you I was leaving in 10 minutes’ - I impulsively got mad.
even though I raised my tone and volume I wouldn’t call it yelling but it clearly caught her off guard, and said that’s not up to her to leave my kid here unattended (pretty shocked at that), nor was it ok that earlier deciding that if it’s ok with my child (rather than asking me) that she could stay while I worked.
And she then said she had mentioned leaving her (and I said well I didn’t hear that - this is where I start to doubt myself because the ADHD makes it hard to recall) to which she said ‘yeah you didn’t!’ In an equally angry way, and instead of continuing the discussion stormed off- then I said ‘ok ‘mum’ walk off then - because conflict avoidance is something she’s terrible at

I think we both misunderstood each other a lot and although I was quick to anger I don’t think it was without merit. I feel as though when things inconvenience her she is happy to not help out as much or take an easier path, and it feels that is coming at my expense (like allowing my daughter to stay here when I’m working which ultimately leaves her in my care, which is the whole point of having someone else do it while working)

when crossing paths next I said let’s have a cup of tea later and we can chat. To which she said there’s nothing to talk about really she had things to do and I had to work.
she’s now gone elsewhere for the evening and seems to be avoiding me.

I can’t let issues stand I have to talk them out, she seems to want to give me the cold shoulder - but then o have to put up with this in my own home.
how do I get this resolved, knowing my RSD and ADHD will make it hard for me to stay level in an argument.

the other hard part is that because of the way I reacted I will be the bad guy. I want to apologise for getting mad the way I did but without pretending like there aren’t any other issues at play.
I feel often like the bad guy with my family and I also won’t be surprised if she’s at my mums house currently who has previously told my sisters to ignore me and does not support resolutions.