What should the sister do upon discovering that her husband is talking to other girls, and confirming that, and her not feeling safe with him while she is pregnant?

If your sister's husband is as you described, he is committing a grave evil. We advise your sister to be patient and supplicate for his guidance and repentance, and to counsel him with wisdom and good admonition. If he repents, then praise be to Allah. If not, she should be patient as much as she can. If her situation becomes unbearable, she has the right to seek divorce due to his transgression and corruption of his religion, while considering whether divorce is in her best interest or not. She must also be careful not to spy on him, as this is forbidden in Islamic law.