Is it obligatory for the wife to obey her husband in going to the doctor because of her thinness, and is the husband sinful for asking her to do so, and was it permissible for him not to marry her for this reason?

You bear no sin in asking your wife to go to a doctor to find out the cause of her thinness and to treat it, but she is not obligated to obey you in this. There should be understanding between you, and she should not refuse something that pleases you and does not harm her. You could have refrained from marrying her due to her thinness, but occupy yourself now with what benefits you, and be keen on what benefits you, seek help from Allah, and do not despair.