Looking for advice: Brother's shih tzu has become aggressive toward dogs since older dog died

Hi all,

My brother and sister-in-law had three shih tzus (older female, male, younger male, all de-sexed) who were always friendly and got along with our dogs very well. The older male liked to hang out on a long leash for about 20 minutes each day, sitting on the grass on their front lawn and watching the world go by.

While his older brother was sick and dying, Petey was very subdued and didn't eat much and was very deferential to the older one. When the older male was euthanized, Petey started to want to sit outside - something he'd not usually done. Recently, he's become aggressive and attacked another dog outside twice. This was a dog he knows and previously had no issue with. My sister-in-law said she cannot understand it, unless Petey feels like he now has to "protect" the house. She said that he was always the dominant one, but perhaps Petey thought that the older male was. She talked to the neighbor tonight and told her Petey would not be outside anymore unless it was in their fenced backyard.

They are coming to visit next week and of course we'll be cautious, especially since we have a new male Petey hasn't met yet. Any suggestions as to how to introduce them (other than the usual "introduce them outside of the house or on neutral turf"?) Can anyone guess what's happened to Petey now?

I will crate my dogs if I need to, and she will do the same with Petey.