I thought Pterano was her brother since he's referred to as Petrie's uncle.
I remember that episode well. Petrie's 3 siblings (guess the rest were out having a coffee break during this episode) were always terrified when Chomper merely looked their way, except towards the end. Makes me wonder how Ducky and Spike's siblings would react.
As for her being swayed by Threehorn, it seems everyone is either bullied or swayed by him, luckily his plot device powers that the script writers give to him. The only ones that seem immune of the residents are Grandpa & Grandma Longneck, and Mr. Thicknose.
As for keeping him away from her kids, as I recall she wasn't to far away so likely could hear what he was saying. I'd guess she would not mind him telling her kids a story, but if he started to try to get them to do certain things she'd likely have come out if she could have heard what he was saying. Though I've not see the movie in a while so may not remember some details, and I'll watch in in about 7 weeks roughly. I think when he asked Petrie do to something I think he had to whisper, maybe, so she would not hear.