The day formula stored for this lot is Ascendant plus Jupiter minus Saturn. It uses the same two planets as the Lot of Children and uses them the other way round, Jupiter added and Saturn subtracted rather than the reverse.
That has a precise consequence. The two lots are equidistant from the Ascendant on opposite sides: whatever arc separates the Lot of Children from the horizon, the Lot of Siblings sits the same arc away in the other direction. They are reflections of each other in the Ascendant. An astrologer who has computed one has, without further work, computed the other.
The motion is the Ascendant's, again almost exactly, because Jupiter and Saturn between them contribute about three minutes of arc a day. One degree every four minutes, a whole sign in two hours, and no usable degree from an approximate birth time.
The Jupiter and Saturn elongation embedded in the formula returns to the same value at every conjunction of the two planets, about every twenty years, so everybody born within a few years of each other carries very nearly the same planetary component. As with the Lot of Children, the birth time supplies essentially all of the individual variation, and the doctrinal reading of the lot as describing a person's brothers and sisters rests on a quantity that a whole birth cohort shares.