Is it better for me to pray alone, or should I wait for my sister so we can pray together in congregation, knowing that I cannot tolerate delaying the prayer past its prescribed time?

Performing the prayer at any part of the time is permissible, and congregational prayer for women is recommended (mustahabb). Therefore, delaying the prayer to perform it in congregation is preferred over attaining the virtue of praying at the very beginning of the time, especially if the delay is slight.