AT 30 weeks the baby's main aim is to put on weight, most of his/her organs are developed and the last trimester is the weight gain trimester. If the weight gain and growth is being compromised then delivery may be the best option.
Significant progress has been made in the care of the preterm baby, at this gestational age
the baby will still be developing his/her full lung function and this may be compromised, for this reason he or she will likely require oxygen
For you sister in law the doctors may be worried about pre-eclampsia or eclampsia, the only cure for this is delivery of the baby.
I hope everything works out well for you sister in law and the baby