Is it permissible for a girl to stay with her married friend in her spacious home to help her before childbirth?

There is no harm in merely residing with your sister if you are safe from temptation and if no forbidden religious act results from it, such as impermissible seclusion with her husband, or forbidden mixing, and so on.

However, it is not permissible for you to travel to her if she is in a country other than yours, and your going to her necessitates travel without a mahram.