Hey @Amaris,
Your whole post and intentions show how much you care about your parents, despite their mistakes and the hurt they might have been causing around them. I see a lot of myself in the way you observe and understand each member of your family. I’ve always been in this position too - and always the emotional pillar for each member as a result. Somehow, I never asked to be in this position, and at the same time I reinforced it by trying to take care of them all too many times.
I don’t know if there is a right or wrong way to approach this as it is about things that are painful and affecting everyone in your family one way or another. It involves deep emotions and others reactions are impossible to predict with certainty. However, it is absolutely true that, as the child, your role is not to find solutions for your parents and be almost their therapist (not saying it in a judgmental way). They have to figure out the steps they’d be willing to take by themselves, which would never prevent you from saying “I would like for you to be helped and to consider this possibility” either.
For now, there’s this heavy secret and burden on your shoulders, and despite it all you manage to think about them, how they feel, how they would react and what could be good for them. I think your parents are very lucky to have a child like you. They may not be aware of it because of all the things that have yet to be discussed between you all, but I see how much you care and you truly have a loving, caring, beautiful spirit.
What I would like to encourage you to consider more than anything, is you. The urge to talk and find a solution, any solution, comes down to the hurt you feel and a need for it to be expressed. It’s a very personal opinion, but it sounds that, at this point, your parents need to hear what you’re aware of and how you feel about it. I think they need to be aware that there are things at play that can’t stay as such. Secrets like these can’t be hidden forever, and the cost on you and your brother is unfair. They both need to be faced with their responsibility of parents, by addressing their couple issues on their own and, as a consequence, having more room for being your parents too. Conveying that can be done in a loving and understanding way, just as you’ve displayed in your first post. However, the conversations you intend to have - no matter how, no matter with which parent - are likely to be painful/difficult, so I hope you will keep in mind to do this for you first and foremost. You too have a voice to use and a heart to be cared for.
What is on your heart, Amaris, is certainly what is going to guide you in the way you would initiate these conversations. And regardless of their reactions, you cannot be wrong for expressing how you feel about a situation. It’s just how it is. 
