Advice needed. My dog sliced open my other dogs ear and it required stitches.

I have a 3 year old blue heeler mix. He's aggressive to others that he doesn't know and has bitten 2 people that have come onto our property unannounced. We've been working on training him with an e-collar at sit means sit over the past few months. He's come a long ways with training and I'm taking him on long walks with people and other dogs (the only issues now are of him lunging at bicyclists, but he's getting better at this with more exposure). I muzzle him anytime he's outside our property on walks. Since I've been working daily with him, I've found myself falling in love with him more everyday as he's now become pretty attached to me and enjoys our adventures and our training sessions. He's become a pretty good boy. We still have issues with delivery drivers that come to our house. He does show some aggression towards my other dogs, but only at the door when a delivery driver is outside and there getting all riled up. I've never seen him be aggressive to my other dogs except at the front door, but sometimes they play pretty rough. But I never thought their rough play was aggressive.
Last night I noticed my black Labrador was really licking her paw and wiping her face. I went up to her and investigated and found two gashes on her ear. One was deep and I knew it was going to require stitches. So I took her to the emergency vet and brought her home late last night. (We live on a lot of property and have a dog door, we also have an invisible fence line). I was not home when this happened. Ever since this dog was a puppy he's pulled on my labs ears. It's just something they do when they play and I've always been worried that he was gonna break the skin, but hasn't. My lab definately stands her ground if he's getting too excited and she doesn't want to play. After speaking to my family they think I should put him to sleep. They told me it's only going to get worse from here. My husband doesn't agree with this, but I've never fully trusted my blue heeler either (that's changing slowly with training). I will never know if this was out of aggression or an accident during play. My initial thought was they played too rough because I've never seen them be aggressive toward eachother, but after speaking to my family I'm starting to question my initial thoughts. I also should add, my parents have always had pit bulls. They had to do a BE because they came home to their pit bull covered in blood, it had killed their other dog. This pit bull was always really aggressive to people and other dogs. It never got better only worse for their dog. With the 2 bites to people and now this, I'm just at a loss of what to do. Any thoughts or advice would be greatly appreciated.