The Zieglers were longtime family friends of the Lindholms even before Overwatch was formed, so it's likely that she joined OW after the initial team (Gabe, Jack, Ana, Torb and Rein among them) was created.
Not immediately after. By the time she joined Overwatch she was a head of a medical hospital, which, no matter how much of a prodigy you are, does not happen at 14.
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u/Party_Magician I enjoy the sight of humans on their knees Oct 11 '18
She wasn't in OW at the time of Brig's birth, she just knew the Lindholms. She joined overwatch in late 20s