I feel your pain. Even when you say you couldn't cry and you blame yourself for this, you were full of pain and disbelieve... Hard to expect something like this may happen to your loved one, even for adult, but for a child whose mom choose such a horrible way to leave this world is just beyond imagination. I probably would have same reaction and tears would come later... Everyone is different. You don't blame yourself ever, this is not your fault, this is not something you choose to go through. Your mom did.
Why? Sweetie, adult world has a lot of ups and downs, not easy. Some people stronger than others, they can go through hardships with a smile and some people can't go... I personally dont think this has something to do with your uncle, I think your dad may open your eyes on this situation later in life, he maybe afraid you are too young to understand. But when you grow older, you ask him questions and hopefully he will cover all the answers for you. In order for you to go on and not to feel guilty and wondering WHY? you must get all answers.
I knew a teacher in our school who was so loved by all students; one day she didn't come to work and we heard she took her own life. She had wonderful life, she had so much to live for and all of a sudden this... And only b/c she found out that she had 4th stage ovarian cancer. See, most people would try to fight it, but she couldn't and preferred to go this way instead of prolonging her misery.
You mentioned that you had a good mom, she would never hurt her kids. What surprised me is the fact that your 2 years old brother was left alone while she was in her bedroom. Kind of strange for a mom to do that. I understand your curiosity - who wouldn't want to know details? Someone in your family must know the truth and I am sure one day you will know it all. Your mom could tell you she doesn't believe in depression, but this doesn't mean she didn't have it.
Many people, even severely depressed, don't want to go on medication, they in denial about their health issues. Personally, take it from a person who is involved in medicine, I do believe that your mom could suffer from depression; most suicidal patients have some type of mental/psychological issues.
Just know that every experience in life make us stronger; unfortunately you had to go through such a scary and tragic event too early. But I am pretty sure you are a strong person and can be a great support for your siblings and even dad. Just please, don't blame yourself for being a bad or ungrateful child. We all deal with grieve differently and if you don't cry, doesn't mean you don't suffer inside your little soul...
Hugs and best wishes to you and your family