Jealousy
Marissa focused all her attention on her dying husband and children. She was very careful to avoid Ella at every cost. She knew that if her path crossed with that of Ella, all hell will be let loose.
Even when Marissa found out about Rex's request for Ella to be serving his special tea every morning and evening, she didn't make a fuss about it in the open due to Rex's depreciating health.
She feared that Rex would die almost immediately if she involved herself in a catfight with Ella over him. Hence, whenever she was with Rex, she bottled up all her suffocating emotions deep within her heart.
However, in the privacy of her bedroom, Marissa gnashed her teeth in ire and rained invisible fire and brimstone on Ella. She sometimes cried herself to sleep whenever Ella and Marissa spent a longer time in Rex's room.