If I were you, I would tell the 大女 that you are having a baby inside 肚肚, hold her hand to touch the 肚肚, let her also feel the baby. She must loves it.
Also tell her that she becomes an elder sister soon, she will have to teach and take care of the baby TOGETHER. Give her some responsibility, your 大女 will understand gradually.
Relax, and also teach her that you cannot 抱 her in these months.