Peter and Fudge's mom, Ann Hatcher, has her hands full with two boys, especially when one of them is Fudge. She ropes Peter into helping take care of Fudge and trying to convince him to behave. It's only because she appreciates that Peter's so calm and responsible, but she really doesn't tell him that enough. She's too busy tearing her hair out over her younger son. She also tends to be overprotective of Fudge, fussing over him at the slightest thing.
Despite the fact that Fudge is a little terror, Mrs. Hatcher tends to take her younger son's side in conflicts, at least from Peter's perspective. For example, when Fudge pretends to be a bird and knocks out two teeth jumping off the jungle gym, Mrs. Hatcher blames Peter instead of Fudge or Sheila (who was the one who had volunteered to babysit in the first place):
My mother raised her voice. "I left your brother with you for ten minutes and just look at what happened. I'm disgusted with you."
"It was Sheila's fault," I said. "You said Sheila was in charge. So how come you're mad at me and not at Sheila?"
"I just am." my mother shouted. (4.95-97)
Mrs. Hatcher's not trying to be unfair. She's just stressed out and needs to take out that stress on someone. She knows that Fudge is really too young to understand what he's doing a lot of the time, and she always comes around and sees Peter's side of things later on. After the playground incident has blown over, Mrs. Hatcher comes to Peter and apologizes:
"Peter, I said some things yesterday that I didn't really mean."
I looked at her. "Honest?" I asked.
"Yes… you see… I was very upset over Fudge's accident and I had to blame somebody. So I picked on you." (4.103-105)
At the end of the day, despite freaking out on a regular basis, Mrs. Hatcher is still a great mom. She cares deeply for both of her children, and she makes it her main priority to ensure that they both feel loved, protected, and cared for. Like Peter, we'll forgive her for worrying just a little too much about Fudge.