My sister gets these itchy red hives after she takes a hot shower. i was wondering if anyone could tell me what it could possibly be and if there is some type of over-the-counter cream or medicine she can use that would help with these.

Cholinergic urticari: This seems to be cholinergic urticaria. She could try taking some otc antihistaminics which may help. If that does not help then she should see a dermatologist.

See an allergist: It sounds like your sister has urticaria, which is another word for hives. There are many different types of urticaria, and many of these are triggered by physical stimuli (heat, cold, pressure, water, etc.). The most common "type" of urticaria is idiopathic urticaria, which are hives without a clear cause. Your sister may have cholinergic urticaria, which are small hives triggered by heat/sweat.

Try tepid showers: As stated your sister has cholnergic urticaria brought on by the hot showers. Instead of hot showers she might try taking tepid showers instead, i.e. Reduce the temperature of the water.