An English writer born in France and grown up there for the first ten years of his life and also having died there, who possibly spoke and wrote French even better than English, puts this expression in the mouth of a young woman in one of his short stories in response to a question she apparently found intrusive, and goes on to write:

“This phrase, which of course means: and your sister, and sounds harmless enough, and even pointless, is a trifle vulgar and is used by well-brought-up young women, I think, only if they want to shock. It expresses the most forcible unbelief, and the only correct translation into the vernacular is too coarse for my chaste pen.”

The CNRTL does not however mark it as "vulgaire" :

« Loc. Et ta sœur! [Exclam. railleuse pour mettre fin à des propos jugés inadmissibles et qui s'attire parfois la réplique non moins narquoise Elle bat le beurre!] »

https://cnrtl.fr/definition/s%C5%93ur