Lots of people on this forum suffer from mental illnesses and physical ailments and we all know people in real life who suffer tremendously. Yet they (most of them) aren’t treating their loved ones in this appalling manner.

She does it because she can. If you continue worshipping her and serving her hand and foot and put up with her verbal abuse and her lack of appreciation, she’ll continue sitting around and being mean. Because she has zero incentive to stop.