I dont recall any scene implying that she cheated on him with his brother. if there was such a scene that made u think that way please remind me. i dont think she has any reason to cheat on him bcz if she doesnt love him enough then she wouldnt get engaged to him considering his disability right? if she liked the brother then no one is forcing her to marry the disabled one right? so no need to get engaged and then go behind his back as he is not some rich guy.
and is it normal to be engaged to a mentally challenged man? well it maynot happen often but we see odd couples all the time like there may be huge age gap or other kind of disabilities. for example, she might have had trauma with other men that she finds him safe, just a case. many reasons for her to choose him.