Twenty-year-old Erika had upset the rankings to win their only previous professional meeting comfortably in Wuhan last year.
She broke the serve of 17-year-old Mirra, the world No.7, in the opening game but from there the contest swung firmly in the favour of the Indian Wells champion, with Erika, 90 places below her sister in the world rankings, taking an early medical timeout for treatment to her right knee.
Trailing 6-2 1-0, Erika decided she could no longer continue, sharing an embrace with Mirra, who will now take on fellow Russian Ekaterina Alexandrova in the second round.
"We share the same room, so of course I knew what she's been going through," said Mirra in her on-court interview.
"Honestly, I have weird emotions. I should be a little bit happy that I won the match but still I'm super sad it ended like that. I'm sure she's going to take her time and recover and she's going to come back as strong as she can."
World No.2 and two-time Stuttgart winner Iga Swiatek started her clay-court campaign with a 6-2 6-2 win over Jana Fett.
The four-time French Open champion took one hour and 16 minutes to ease past her Croatian opponent, who produced her ninth double fault of the contest on Swiatek's second match point.
It was Swiatek's first game after she was verbally abused by a man while training at the Miami Open in March.
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