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.
Your mother-in-law has no right in preventing her daughter who is your wife from becoming pregnant and from having children, and your wife is not obliged to seek her permission about that and neither is she obliged to obey her in this regard. Obedience is required only in what is permissible as the Prophet informed us. No doubt, her preventing her daughter from having children is not within what is permissible.
As regards your wife’s abortion, if your mother-in-law made your wife have an abortion or ordered her to do so, then she is sinful. Aborting a fetus is forbidden even if it is at the Nutfah stage according to the view of the majority of the scholars . For more benefit, please refer to Fataawa 85970 and 85558.
Moreover, if this abortion took place after the fetus had taken the shape of a human being (40 days and above), then the one who carried out the abortion is obliged to pay a Diyah (blood money) for the child and it is one-tenth of the Diyah of his mother, and it will be divided on the heirs of the child who did not participate in aborting him.
Finally, if your wife had obeyed your mother in aborting the child, she is obliged to repent to Allaah; for more benefit, please refer to Fatwa 87903.
Allaah Knows best.