How should I talk to a parent about psychological trauma they have caused me and my siblings?

I sympathize with your situation. I have dealt with the same issue, but with both of my parents. I have chosen that discussing it with them would serve no purpose. As a parent myself now, I also see things differently. Although I don't agree with how I was raised, my parents raised me and my 3 brothers to the best of their ability. Even if their ability was lacking in many areas. I no longer hold them accountable for my issues. Rather, I now know the root cause and hold myself accountable for choosing to succumb to it or rise above. That being said, you are in a difficult situation as you are caught in the middle now that your parent is aware of the dialogue between you and your sibling. Not an envious spot. How do you talk to your parent about this? This all depends on the parent and how rational or open to a possible discussion he/she may be. You will need to choose whether the discussion will have positive or negative based on your intimate knowledge of your parent. I did sit down with both my parents once several years ago and talked with them about some of the issues I was dealing with at the time. Rather than take the stance of accusing them of poor parenting, I opted to tell them what I was dealing with at the time and how some of their decisions made me feel growing up. I talked about my feelings. I pointed no blame. To be honest, I was surprised at how shocked and apologetic their reaction was. I thought for sure I would be facing harsh criticism and defensive arguing, but I got the opposite. I still have not brought up a lot of what I saw as their shortcomings as parents. Who is ever receptive to that? It would do more harm than good. I have accepted them for who they are and try to be a much better parent to my children. But you never know the reaction you will get by being honest and talking about things from the perspective of how they made/make you feel. Don't point fingers. You may find a closer relationship, in the end. None of us are perfect. Although we may feel like we have the right, who are we to judge? We are all fallible human beings. I wish you the best of luck in this. I sincerely hope for the best outcome if you choose to pursue this.

Sorry for the delay of my answer, as you invited me to answer this question a few days ago. This delay...it was not because I didn't think at this issue, but because I thought a lot about how this matter could be addressed. Urrr... this is difficult, as usually everyone who gives an advice uses his/her own experiences in the relationship with his/her parents and judges the facts according to his/her perspective. It's sad, but in fact no one can step into your shoes. You passed through an unique experience. Your relationship with your parents is not like any other relation. I think that even the definition of "trauma" is different for everyone of us even if some people would think that they could properly understand what you mean by "trauma".  You are the one who knows what it works better in the relationship with your parents. Just ask yourself and follow your instincts. Sorry, I really wanted to help more than I did it, but I couldn't. I'm sure that you are a powerful person, as you succeeded to overpass through a difficult experience.  I'm sure that you are a wonderful person, as you are thinking so much at your sibling life and you are looking for the best way to help her. So, I'm very confident that you will succed :).

I once did that! To tell the truth, I also feel that part of my personality is formed by the constant quarrels between my parents. When I am aware of this, one day when they quarreled again, I seriously said to my mum how their quarrel influences me in a bad way. She said nothing, but since then, although they still quarreled, but I could feel that they usually avoided doing that in front of me. ( meanwhile, it means they still quarreled...)

Clearly. Ideally, you try to be as rational as possible, but since you were a victim, that may be impossible. Keeping your goal in mind of helping your younger sibling may help to keep emotions at bay when you speak to the parent. Emotions are good, important, and they will pop up in this discussion. They can also sabotage your goal.

Straightforwardly. I don't think things will get better otherwise.

ok . you need to get two things straight. first, no matter what you've seen in life, even maybe because of your parents you cannot blame your depression. the worst thing to do is blame people for mishappenings in your life. second. at the end of the day , they are your parents. no matter what they do they want best for you. forgive me if i'm wrong . or maybe the question skips a few details as to what kind of trauma you have been through. now as for your question, you need to pool your family and pull your shit together. sit out straight and discuss everything there is to be. point blank, no drama. be polite and yet if there is something you want the other person to do , be very firm about it. needless you remind you, this is your family we are talking about. let each other know what you want and expect from the other person. or don't want ad don't expect. whatever. just blurt it out. be as honest as you can. believe me, their side of story will be as convincing as yours is. thats the best i can suggest. please also try and understand that your parents probably deserve better after all they've probably been through. maybe they weren't perfect. hell you'll know that once you're a parent yourself. i bet none of it was intentional, but if you sit and discuss it through, the moment they know they've been at fault they will do their best to make things better for you. just be kind and yet firm. i really hope this helps !