Toddler pulling sisters hair out

This might sound stupid to some. But our toddler keeps pulling his bigger sisters(4) hair out in big chunks.....and not only when they argue, he truly does it at random. I feel so sorry for her. I even put a cap on her head at home for a week now. But she is quite sensitive and doesn't want it on, or even in a ponytail/ knot..

We tried everything to stop him from doing it, but he keeps doing this at least twice a day. It's gotten to the point that his sister now is getting bald spots... Is there anyone who has experienced this, and what did you do to prevent it from happening so I might see some other tricks / tips I haven't tried yet with him.