Hi, here mother helding a suspecious thought in a predominant manner, which affecting her familial relationship significantly especially causing interpersonal distress. Despite her transient think of non existence of infidelity idea after convincing by you and your father, it will resurfacing to her mind. Hence, it causing influence of her thinking, emotionality, and behaviour. It's clinically warrant to intervention condition called persistent delusional disorder, a jelous type.
Next Steps
Fisrts step, she required to assessed comprehensively by a psychiatrist and his team( past and family hisory of psychosis spectrum condutions and other psychiatric and personality disorders).
second step, she needed to start evidence based antipsychotic medicine by the treating psychiatrist which is indicated according to her other medical conditions.
Imporrantly, she needed to encouraged think about automatic negative thoughts for infedility and how to gather contradict evidence to weaken her overvalued idea of suspecion by cognitive behaviour therapy model.
Health Tips
Consult Psychiatrist as early as possible before acccumulating negative consequences of its result.
Extend her support by family members.
Avoid isolation and encourage to cultural activities and socializing works.