No such thing.
Even in you were metaphorical "twins" (born on the same day) there is still going to be one of you who kneeled and presented tea before the other one. Even if it was only earlier than you by one minute, that person is your shixiong and you are the shidi (Cantonese: Sihing and Sidai)
Don't forget that traditionally "seniority" is the same in Kungfu as in the workplace. It has nothing to do with rank or belts or what you've learned. It's simply a measure of how long you have been Sifu's student.
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Interestingly, in northern China...well in Xi'an anyways...instead of Simu (师母) the term of preference is (Mandarin) Shiniang (师娘). It's an oddly old fashioned term as "niang"/娘 is a term for "mother" that you don't hear in contemporary Chinese. You usually only hear it in period pieces on TV.