Please note that you have not gived a definition for your numbers. 15,000 platelets per µL? mL? L? It matters. 50,000/L is much different than 50,000/µL.
Thrombocytopenia is not uncommon during pregnancy, and, as in non-pregnancy related cases, results from diverse causes. Without awareness of the cause(s) (i.e. knowing which tests she has undergone and the results), no one here can recommend the proper management of your sister's case.
Some causes of thrombocytopenia are unique to pregnancy and may not be familiar to hematologists.
Incidental thrombocytopenia of pregnancy, usually referred to as gestational thrombocytopenia, accounts for 70%-80% of cases. It occurs in the mid-second to third trimester, and its pathogenesis is unclear. It has been speculated that it may result from various mechanisms, including hemodilution and accelerated clearance. No confirmatory laboratory tests are available, and the diagnosis is one of exclusion. ...[However] we consider a diagnosis of gestational thrombocytopenia unlikely if the platelet count is < 50 × 109/L, with very few cases having been described with counts 40-50 × 109/L.
ITP is not an indication for cesarean delivery. Mode of delivery in a pregnant patient with ITP is based on obstetric indications, with avoidance of procedures associated with increased hemorrhagic risk to the fetus (eg, forceps, vacuum extraction, and fetal scalp electrode/samples).
As a rule of thumb, developing a platelet count < 100 × 109/L early in pregnancy, with declining platelet counts as gestation progresses, is most consistent with ITP. ...Nevertheless, knowing the exact diagnosis at that stage of pregnancy changes the management very little, as will be discussed in the indications for treatment.
A rare inherited cause of thrombocytopenia is type IIB von Willebrand disease (VWD). Women with this condition may develop thrombocytopenia, for the first time, in pregnancy and be misdiagnosed with ITP. Platelet counts may occasionally fall to levels as low as 10-20 × 109/L at term, typically with nadir value 1-3 days before delivery, but they rapidly improve after delivery.
Below are three links to the American Society of Hematology journal, blood. There are linked references within the articles that can assist you in addressing your concerns with her doctors.
Also, your hospital should have a medical library, where you might be able to access medical information with permission.
How I treat thrombocytopenia in pregnancy
The American Society of Hematology 2011 evidence-based practice guideline for immune thrombocytopenia - note: this is for immune thrombocytopenia
Standardization of terminology, definitions and outcome criteria in immune thrombocytopenic purpura of adults and children: report from an international working group - note: this is for immune thrombocytopenia
A rational approach to the diagnosis and management of thrombocytopenia in the hospitalized patient.
Assessment of thrombocytopenia