No, there is nothing unusual in this behavior. Simply re-state the rule and let him know that he can expect a time out if this occurs again. If he does it again, place him in his time out chair for 3-5 minutes. Track the time with a portable cooking timer, and tell him that, during the time out, he is to remain seated and quiet. When the time out is over, ask him what got him in time out and reiterate that he is not to encourage his brother to play in the bathroom.
Typical behavior...no yelling needed. if he brings his sibling in the bathroom calmly lead him to his time out. From not yelling, he'll learn no one gets worked up from his teasing and potty talk, and the time outs won't be worth it. if time outs don't work, take away a favorite toy or activity.
("Potty talk" also normal. )
A good idea at this age - fill a box with plain rice for kids to play with like a sand box - put towels down in the bathroom and let them play with water toys in the sink. This will fulfill that tactile play preschoolers enjoy.