Complex sentence: By the time she was 12 his sister was able to pin her 20 year old brother in a matter of seconds in wrestling matches and beat him in arm wrestling and she had no interest in challenging his older brother or getting trained by him.
Split into simpler sentences: By the time she was 12 his sister was able to pin her brother in a matter of seconds in wrestling matches. Due to her superior skills and strength that point she was no longer interested in challenging her 20 year old brother or getting trained by him.