Is it permissible for a wife to pour her milk on her husband's body with her consent, and what should she do if the husband requests that and she is not consenting, and would she be sinful if she does not obey him?

There is no objection to performing this act. The general rule regarding such matters is permissibility, unless there is evidence of prohibition. If the husband requests it, it is preferable to obey him, provided no harm results from it. However, if the wife refuses, the husband is not permitted to compel her, and she does not sin by disagreeing with him.