As-Salamu Alaikum,
May Allah subhana wa ta'ala reward you for your efforts. A younger sister who was a minor married an older brother in America. Due to the laws, if she was to get pregnant he could be placed in prison. Her mother is a disbeliever and will try to harm her husband because she does not like him. If she missed her period and her and her husband thought she was pregnant but wasn't sure, and he ejaculated in her the second time because of the uncertainty, than would it be permissible for her to get an abortion for his safety?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Abortion before the soul is breathed into the fetus and exactly during the Nutfah (sperm-drop) stage is permissible in the above case that is mentioned in the question. Since not aborting Nutfah at this stage will cause great harm to the mother or to the father as you mentioned in the question, then it is permissible to abort it.

For more benefit on having sexual intercourse with a pregnant wife, please refer to Fataawa 82144 and 84328.

Also, for more benefit on residing in non-Muslim countries, please refer to Fatwa 86405.

Allaah Knows best.