Can An Adult Child Testify Against Parent A For Parent B?

Hello, my parent (B) has finalized divorce to my parent (A), but both of us are concerned about my two little siblings (C and D) about their well-being and safety at A’s household. B went 50/50 on custody because A’s parent (E) helps care for C and D while it is A’s week and wanted to give A the chance to step up. A have not stepped up and instead leaves E to care for them. E is quite old and showing signs of mental decline, and all of E’s children including A rely of them for money. A struggles to consistently go to work and has a problem with drugs ranging from alcohol to pills to MJ and even Coke, and a history of DUI’s including a jail sentence on one account followed by driving privileges lost. Even at A’s household one of their siblings lives there as well and leaves pills, mushrooms, and alcohol easily available and is often not sober to notice. A has also been abusive in the past to both me and B, until we moved into E’s house before C or D could remember, where the physical abuse stopped and the mental abuse continued in a narcissist pattern. I have seen first hand the damages they can do and have been forced to listen to their complaints of B and C and D for years.

I want to know if my account of the past would help seal the deal and give B the full custody of the kids. I am a legal adult and was already an adult when the divorce finalized. B has lots of money and their own accounts to give as well but I want to know if mine would help B in court. We have not formally begun the process as we have not noticed immediate signs of abuse other than A bad mouthing and manipulating C and D. We hesitate because both are in school and we are currently in an awkward place schedule wise to assure both get to school or on the bus safely.

Any advice would be greatly appreciated, B does not know I have posted this as I believe they have enough to worry about on their plate currently.