A question for the parents

I am not sure if this is the right forum to be submitting this but I will give it a try.

Ok, here is the dilema. I am 24 and moved out of my parents house 4 years ago but have recently found myself living back at my parents house temporarily for two very good reasons. 1. is that I am taking care of my sick mother and 2. is because I left my physically abusive husband and needed to get out of the relationship.

My brother is 21. He has had an on and off relationship with his girlfriend for about 2 years. When he started dating her he was 19 and she was 16. She is now almost 18 years old. She is a nice girl but a complete air head. She's a spoiled little girl. They fight constantly. She cries everyday. They do drugs together and spend hours upon hours sitting around in his room either having sex, doing drugs, smoking, or eating. She can be EXTREAMLY annoying. All I hear all day long is her screeching voice complaining about everything on the planet. What my brother sees in her is beyond me. It's not that I don't like her...I just don't think they are right for each other. She is of the age where she thinks that my brother is the only one for her and she will never meet anyone else.

The thing about my brother is that he does drugs, doesn't have a job, sits on his ass ALL DAY LONG playing computer RPG games religiously, and for money he either drains my parents account or sells drugs. He has no life and no ambition. He never went to college and didn't graduate from high school but got his GED. He's been arrested, been in jail, been to rehab, been to juvy...you name it. He's all tatted up and has piercings too.

Well...the other day my brother (who confides greatly in me) comes up to me and tells me to check out the "gift" he got for his girlfriend. It just so happened to be a diamond ring...which she wears on her left ring finger. He also has a ring that he wears on his left ring finger. She at first tells me that it is a promise ring but days later she started calling me sister...in law. My brother corrected me when I called him her boyfriend by saying, "fiance". I asked if they had told anyone about this news and they said, "not yet."

Ok...this is just bad. First of all, I got married young too. My marriage failed miserably and I don't want the same thing to happen to him. I believe deep in my heart that he is making a BIG mistake. My parents are going through SOOO much crap right now, the last thing they need is my brother announcing his engagement to a child.

So my question is this...should I tell at least my dad about what I know or wait until the s%@t storm comes at him first? If I tell him, my dad won't tell my brother that I told him, but he will have the "talk" with him on his own. I am really worried about what will happen if my brother tells him and my dad is completely unprepaired for it. My dad is at the end of his rope already with my moms illness, my divorce, and my brothers constant run-in's with the law. What I really want to tell my dad is basically "I think you should have a talk with my brother about where his relationship with his girlfriend is going. No questions asked."

What should I do?