Page 82 says a Dedicated Transport in reserves can only begin with the unit it was selected with embarked upon it plus Independent Characters, page 135 says units embarked on a transport in reserve must arrive together, nowhere in the rules does it support the premise of units moving in and out of transports in reserve in fact the exact opposite. Once the vehicle arrives on the battlefield units may enter the dedicated transport if it has sufficient room to allow for such as indicated in the last blurb of page 82 for the new Dedicated Transports rule.
If you wanna put Sisters in a drop pod create an unbound list or select Drop Pods as a Fast attack option for an allied Blood Angel or Space Wolf force.