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poshgirl, you are very kind to be caring for your mother and you are certainly justified in feeling frustrated. Her pitting siblings against each other and playing favorites is really unfortunate, a really bad way to parent.
PC is a place to vent and we don’t always have to be so politically correct. They are your feelings.
Sometimes children care for their aging parents and essentially get paid by inheritance or similar, so there is a give and take. All give and no take is not healthy for you, Posh. I hope you are not just letting her take advantage and not even be nice to you. You have a life to live for yourself as well. Hugs.