Hey Rebecca, Boy have I been there. I think you could try the letter-writing option; if nothing else, it might make you feel better to air your feelings and concerns and feel that you've done what you can.
But if your dad is an alcoholic, I wouldn't expect too much from his response; one of the biggest components of alcoholism is denial, and it can be a real beast to deal with. If your dad's in denial that his drinking has gotten out of control, then he's spending a lot of mental energy "proving" to himself that he's fine, and your letter may just provoke his defensiveness. If your siblings are also in denial, then it gets really hard because it can feel like everyone's ganging up on you and telling you YOU'RE the crazy one!!
In addition to writing the letter to your dad, I would suggest talking frankly to your siblings. Prepare ahead of time a list of incidents that illustrate your belief that your dad's drinking too much, so that you can "show" them as well as tell them. You might also read up on the definition of alcoholism (one of the key points is that the person doesn't stop drinking even in the face of negative consequences) so you can explain to your sibs why you feel your dad is an alcoholic and the ways in which he fits this diagnosis.
Then hopefully you can have a practical discussion about what to do next. You may not be able to stop your dad from drinking too much -- recovering from alcoholism is a big task and it may too much to expect this late in your father's life. However, perhaps you can tackle some of the other things, such as depression, you feel are contributing to your father's drinking. You can also figure out ways to protect him from some of the dangers of his drinking, such as making sure he can't drive drunk.
Good luck; I honestly feel this is one of the MOST difficult situations we deal with in caring for aging parents!!!!
. He is so anti-establishment about doctors - for a good reason, though I think its stupid to make blanket judgements. ( A couple big doc screwups in the past.)
