Is a sister's intentional abortion of a fetus at three and a half months considered a major sin? And what is required of her for repentance? And must she inform her husband of this?

It is not permissible to abort a fetus after the soul has been breathed into it (at four complete months), unless its continued presence poses a danger to the mother's life. Aborting it at that point is considered an unlawful killing and requires expiation (kaffarah). As for before the soul is breathed into it, scholars have differed on the ruling of abortion. It is not permissible to abort a fetus at any stage without the husband's permission, because he has a right to the child. Therefore, the sister must repent and seek forgiveness for what she has done. She is not required to offer expiation as long as the abortion occurred before four complete months. She must also apologize to her husband and seek his forgiveness, unless she fears a greater harm.