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The reason why it says sister-in-law is because this is a fucking machine translation.
The word has two meanings, the machine picked the wrong one.
(in 99% of the contexts) You wouldn't say "The two girls dressed up to go to the testicle" is the correct interpretation of "The two girls dressed up to go to the ball", would you? Ball has many meanings, you infer which one it is based on context.
MTL (and some people) are stupidly bad at inferring things from context, especially if the context is in a previous sentence.