Sonic would probably be very shocked at first. He trusts Amy, so her betrayal would be unexpected. He might initially try to talk to her to understand why she did it.
Tails would probably be very helpful. He'd use his intelligence to make sure Amy has a comfortable pregnancy, like creating special devices for her. Knuckles might be a bit awkward at first but would ultimately be very protective. He'd stand guard to make sure no harm comes to Amy and the unborn baby.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
Well, Sonic is a hero at heart. So when Amy gets hurt, he would first assess the situation calmly, but inside he'd be worried. He'd make sure she was stable and then look for the cause of her injury. If it was an enemy, he'd go after them. But he'd also be very gentle with Amy, talking to her softly and reassuring her that she'll be okay. He'd do whatever it takes to see her back on her feet again, like finding special herbs or items that could speed up her healing process.