:big hug: I think you know what you need to do!
Let your sister put her children into creche …… call her bluff! You can then take them out and about at your convenience so that you have 'treat' time with them. They may benefit from the creche situation too. Are you in a position to pay part of the week for them? though I wouldn't even be tempted: she had the children she should take responsibility!
What was your response to 'the wrong cornflakes'? I can never think of a suitable retort at the time
but it would run through my head for a long while after! My temptation would be to tell Mother that there are starving kids who would relish the 'wrong' cornflakes ………. but of course, I understand how, when I fancy something, if it isn't what I expected
……….. I would also tell the carers that I never want to have that type of early morning 'phone call, that phone calls should be for emergencies only! and that wasn't such a situation.
Did you put the children to bed that evening when their mother was later than expected? I don't think conversation is required, have a talk with DH and decide together exactly what to tell your sister. He married you, not the rest of your family
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As for the hot weather it makes patches etc. difficult and uncomfortable so time to take some time for U! Unless you step back none of the family will realise what you have to contend with.
What have you decided to do in the interim?