Next time there is a confrontation at the screen door get your Mum (ooops, sorry,....Mom!) to hiss along with your cat. The cat will see this as valuable support, the other cat will think that a huge kitty like that is not worth the effort and all will be happy.

BEFORE doing this make sure Mom lets your cat know she is a friend by gently touching your cat and speaking very gently to it.

Your cat is defending it's space and by taking it away from the situation it is losing ground with the other cats. Your cat doesn't want the neighborhood cats to see it as a wimp and it wants to hold it's territory. So when when removes it from the situation it is going to do whatever it takes to get back in there!!