OMG you poor thing. To lose your beloved and now to have to fight with his parents, it breaks my heart to hear the pain in your words and the frustration which you have EVERY right to.
Inlaws can be horrible sometimes. In their profound grief they might not be as kind as they should be. Which not all people are when they are suffering, some become kinder some not.
You are a good mom. This is your child and I honestly believe your decisions are spot on. Your son needs routine now. He needs a home, a mom and school, and friends. I hope that they will see this and be supportive to you. In no uncertain terms should they try and take your child. Yes they lost theirs, but they can NOT replace him with yours.
Please try and take care of yourself, this is such a stressful time and you carry such a precious package within you.
Hugs and the very best prayers to you and the children.