I use the cream for my psoriasis. today my sister used the cream for her psoriasis-like dermatitis on the scalp from the same tube. is it safe for me now to use it for my scalp or vaginal psoriasis?

Simple rules: Prescribed meds, even topicals, such as creams or ointments should only be used by the person they were prescribed to. Skin conditions may look similar, but may be different and this is where it becomes important not to share. Second, location is important as absorption would be different if applied on the skin or vaginally. If your cream was prescribed to use on scalp, use only on scalp.