I am also a March 2006 mother who is planning to be treated by a midwife rather than a traditional obstetrician. It's very important to me that my pregnancy and birth experience have as little medical intervention as is safely possible. The midwife I will be seeing works with another midwife in an office with three ob-gyns. She also delivers in a hospital. I feel very confident that if a medical situation arose which requires the expertise of a physician, I will be referred to one of the three in her practice.
You can cite her training to your sister, you can cite statistics of how midwife-assisted deliveries are as safe as those of any OB, how their C-section rates, use of vacuum extraction and foreceps is lower, that labors ar often shorter, etc. However, in the end, it's really your decision and your sister should not decide who handles your prenatal care.