Is it permissible for a pregnant woman to supplicate and perform night prayers to ask for provision of twins or a beautiful child, and will her prayer be answered?

A Muslim should supplicate to Allah for all their needs. Allah has commanded supplication and promised to answer, unless the servant supplicates for a sin or the severance of kin ties. Therefore, there is no harm for your sister to supplicate to Allah for whatever she desires, as long as the supplication is legitimate, such as supplicating for twins or a beautiful child. It is more preferable to supplicate for righteous offspring, as the prophets and the servants of the Most Merciful did.