Cats with Unbecoming Dominance Behavior?

We have 6 cats, including a 2 year old mother and 5 siblings that are 1.25 years old, 1 brother and 4 sisters. We are all living in 1 house, with 4 litter boxes (1-2 cats use them).

They are inside to sleep, eat, and at night and outside in the day. There is tons of space outside.

Now, they are getting jealous and fighting with each other. This is quite upsetting to us. We are just a 50+ couple. The 2 sisters started to spray outside recently. All of the cats were spayed/neutered when kittens, at 4 months. All get a 30 minute walk with the wife most days. She is conscientious about treating all fairly.

Some days 2 of them may be bffs. This past week 5 of the cats are bullying and hate the 6th cat who is feeling scared, very insecure, and afraid to go on our walks now. Fights break out when one sniffs another's butt (so annoying for the sniffee), or walking by another or bullying.

Dominance in family and jealousy is what is boils down to, right? What can be done? How long with this last? They used to be so happy, silly, and dumb when they were kittens, not so long ago. When cats are happy, the couple is happy!

By Betcha from Dillsboro, IN