this situation was recently addressed in Catholic Answer (the other one, that used to be edited by Fr. Stravinskas but they needed 3 or 4 priests to replace him) as to 2 lay people who were the only ones coming to Church each day, who were permitted to open the tabernacle, take the ciborium, one communicated the second, then the second communicated the first one. In the situation addressed above, the sister (or any other lay minister) would simply hand the ciborium to the next person, who would communicate her, and then she would take the ciborium and distribute to all the other people.
There is also a very distinct procedure for communion services without a priest, one for weekdays and one for Sundays. It does NOT allow for just opening the tabernacle and taking the reserved hosts. Liturgy of the Word and proper procedures must be followed. These are available from the publishers of sacramentaries and other liturgical texts.