There are many factors that can influence jaundice (high bilirubin levels) in babies. Among the most common are blood type incompatibilities between the baby and mother, dehydration or inadequate feeding, breast milk jaundice, and hereditary factors that affect the natural variation in how a baby metabolizes bilirubin.
Given your daughter's history, it is likely that her obstetrician has already evaluated her for antibodies to blood type incompatibility. Since weight gain has not been a problem previously, dehydration is unlikely.
We still do not understand all the factors in breast milk that can prolong jaundice in some babies. We do know it is still safe to breastfeed even if the baby is jaundiced. If the bilirubin level is high enough to require phototherapy, supplementation with formula is or a brief (24 hours) interruption in breastfeeding is sometimes used to help decrease the level of bilirubin rapidly. Since exclusive breastfeeding is the best way to provide nutrition to a newborn, it is not recommended to start formula supplementation immediately.
Congratulations on all your grandchildren!