Is a suitor's rejection, given that he is poor, married with five children, and the fiancée insists on accepting him, considered right or wrong? And how can the sister be convinced of the unsuitability of this marriage and its harm to the first wife?

It is a daughter's right upon her guardian to marry her off if a suitable man, pleasing in his religion and character, proposes to her and she consents to him. It is not permissible for him to reject him without a legitimate Shar'i reason. The only recognized suitability is in religion and character. It is not permissible for the guardian to prevent his ward from marrying a suitable man; otherwise, he would be unjustly hindering her. If the woman is sane and mature, and willing to marry the suitor, and there is no Shar'i impediment or actual harm, the guardian has no right to prevent her. Rather, he may advise her if he perceives a lack of benefit.