How to react if your mother is having an affair and giving you all her responsibilities over your siblings?

My family is been a messed always, this is not the first time my mom has an affair but before I didn't have to take care of my sister since she wasn't born, but now, my sister even calls me pretend mom, and I hate being around my mom, she would leave at night and sometimes not come back until next day night. Meanwhile my father is just ignoring the situation in order not to discuss my mother and end up in fighting, and he leaves me most of my sister work since, well, it's a girl, and I'm a girl and I should be able to help my sister better than he could due to she still being little.

How to react if your mother is having an affair and giving you all her responsibilities over your siblings?
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