I'm fed up of constant issues between my mum and sister.
My mum likes to be in control, panics and wants the best for her kids more than anything. My sister has her own mind, makes silly mistakes, is easily controlled/submissive (maybe not the right words - I mean agrees easily and doesn't have her own mind) to her partner, struggles quite a bit with social skills so can come across as rude.
There is rarely an all-out fight but my sister is retreating more and more into her own bubble, makes jokes about my mum which are rude and listens a lot to her partner who dislikes my mum greatly.
My mum gets deeply upset that my sister puts on this superior, superficial act to my mum and doesn't really respect her. My mum hates to see my sister agree to everything her partner wants and pick him and his choices, every time, over her family...even if it means double booking or cancelling on mum.
I'm in the middle. Our family is quite broken as it is. My dad left when we were young, my sister is quite similar in personality to him. Mum is perhaps over-sensitive because of it all but I can see it is hard to see her own daughter acting so weird.
Take a step back, they’re both adults and it sounds like your mother has difficulty in accepting that your sister is an adult and she no longer has the level of control she used to. As much as you might sympathise with your mother, your sister has right to her own life, silly mistakes and all.
This is not your fight, don’t let either of them pull you in.