Hi, I'd like to help you with your question.
It sounds as if your grandson might be having a temper tantrum. You did not indicated whether or not this is typical behavior for him but whether it is or not, ignoring a tantrum is one of the best ways to deal with the behavior.
What a tantrum represents is a way for the child to not only express his displeasure about what is going on, but he gets attention as a reward as well. Letting the child express himself in a safe yet out of the way area lets him learn that his behavior will not reward him any attention.
Locking him in his room for a short period might be alright. I would not keep him longer than that.
Once he is calmed, his mother may want to talk with him in a calm yet stern way and let him know that his behavior is not acceptable and if she wants him to apologize to his sister, he should be directed to do so immediately.
Children need to know that no matter their behavior, they are loved. But setting limits and boundaries is important to make sure children understand what kind of behavior is acceptable and which is not.
I hope this helped you,
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