She has had a lifetime of being in control of the family. I think its not as easy as shaping the behaviour of a toddler!!
Her late husband bowed down to everything she wanted. Both siblings said it was her way or no way (though any bad decisions she made for the family would be blamed, for ever, on the late husband).
I have tried going up against her. It doesnt work - or maybe it would for someone that these things effect less (I like a life without bickering!).
Dp is weirdly conditioned to accept her behaviour, and make excuses for it. I have said to him, we can ALL have excuses for selfish behaviour - however - we have to take responsibility for ourselves & our behaviour & no reason is an excuse for bad manners. If I push DP further, he doesnt want to hear it (although, she has stayed till he is exhausted too and he has admitted it really is too long).
I now dont let her control things so much. She used to bring lunch (from Pret or Eat) so could dictate "no, lets not have lunch now, we shall have it after our walk". I now have a perfect excuse with DS - I now can say, well, he needs to have his lunch before 1pm - so I shall do his lunch now and we can meet you on your walk".
She is generous - but controling with it. Same with dinner - I now thank her for the offer of ordering Dominos pizza, but I have bought food to cook . AND - I actually love my break away in the kitchen, DS up on chair "helping" - and I take my sweet time!!! She used to want to stand in our tiny kitchen talking at me - but I did say to her I find it really distracting as I am not a good cook like her and need to concentrate... And also, told DP to keep her out the fucking kitchen!!
She is constantly pushing for her way though. Constantly. The other day, I had prepared lunch (a lot of chopping and preparing). She turned up with a pret lunch. I said - oh - you brought lunch - I said not to this time as I was making lunch - she denied me ever telling her (I showed her the email!!) - which she said she didnt get (but replied to - I didnt push the point).
I am not comfortable with her buying lunch / dinner - because it comes with conditions. Conditions to let her do as she pleases. I have had "you accepted x, y and z from me" - which has made me want to accept less and less from her.