Is the mother sinful and obligated to offer expiation for the death of her fetus and the weakness of the other, given that she did not adhere to all the vitamins and felt tired before the check-up that revealed this?

If it is definitively proven that discontinuing the pills was the direct cause of the fetus's death, then your 'Aqilah (male relatives on your father's side) must pay the blood money (Diyyah) to the heirs of the fetus, excluding yourself. The Diyyah for the fetus is one-tenth of the mother's Diyyah. As for expiation (Kaffarah), it is more prudent to perform it, and it consists of fasting for two consecutive months.