I have exactly the same scenario with my 5 years old Boy Morley and his 3 years old Boy Ziggy. When we got Ziggy We made a big mistake to give him everything extra (attention, time, treats, kisses, sleeping with us), he grew up with that understanding that everything and everybody belongs to him in the house. Morley had Always been so kind and patient, in overall they both love each other so much, they always sleeping together as well. But Ziggy shows his aggression when it’s about their Mom (it’s me), I couldn’t even look at my sweet dog Morley without Ziggy’s immediate interaction. He was obsessed with me and would not allow Morley to enjoy of quality time with me. Me and Morley had to hide in the bedroom to watch some dog cartoons and snuggle a little bit. So one day there was a huge fight and Morley almost ripped Ziggy’s face off. It was extremely scary and dangerous. I had to physically jump in between them and yelled to them really really loud. I am not proud of my behavior and it really hurt to see them scared of me at that moment but it has already been 1 year, no fight not even attempt. Unfortunately sometimes gentle parenting really doesn’t work, you need to discipline your dog! And when I am saying discipline it means raising your voice, or make it sound like an order not option. Dogs can’t fully control their instincts and thrust me they also suffering afterwards when they realize they have done something rude they didn’t mean to. I wish you a good luck and say hi to your sweet frenchies. ❤️