OP so very sorry for all you have experienced.
As others have said. None of this is a reason for you to feel guilty. Your uncle may been 16 but he acted as an adult when he abused you.
There are no excuses for that. Not a rough time, illness etc.
Not all abusers have been victims of abuse nor do they go on to commit abuse. To perpetuate the idea that any reason is an excuse for such actions is utterly wrong.
I am so sorry every adult in this terrible scenario let you down.
I think we can look for reasons and maybe assign labels, maybe your dad was a coward, maybe your grandma was blinded by loyalty for her son, maybe your mum is one of those morally corrupt people who cannot make a moral choice.
Please get more therapy to work through this. I do feel you may need to work through the impact of your family's failure in relation to you in the light of realisation how ugh they have all lied/covered up.
You say you cannot lose your grandma too? If you want her in your life that is your choice. Same with your dad or your mum, your choice.
But you are more aware now, you know what your mum is capable of, she persuaded you not to prosecute. She utterly failed you. If in the future you feel able to go no contact, with guidance from a therapist maybe to help you move forward, please do. If so, please do not waste any more guilt on this family. You have a new family.
Also, you may never really understand why your mum did what she did, it's not about sibling loyalty. I would not support my sibling if they did this. Lots of others have said the same. Your mum made a choice.
Please find your own peace and move forward. Please do not make it a requirement for your peace to understand your mum.
I also feel you are letting your uncle off the hook, mentally, there are no excuses for what he did. Your family has created a culture where what he did was not really wrong enough to warrent punishment, but it was, it really was. There are no excuses for him or your mum or the other adults that failed you. Look after you now. Just you and your family.
Bless you.