I agree that interventions are most helpful when not undertaken by only one family member or friend. Defensiveness can shut out one person pretty effectively, but two, three or more are harder to dismiss. Could the other parent be brought in as a participant in the process? And, ideally, a counselor with experience in this area.
I'm cautious about the usefulness of an email, as opposed to face-to-face conversation. Electronic communication is easier, but impersonal, and can and does trigger "flaming" and e-grudges. There are so many opportunities that an evolving conversation provides, watching facial expressions, body language, hearing tone of voice… all of which could be responded to with more sensitivity than is possible in an email. Even a letter in an envelope, hand-written, would suggest more human caring from you, if you really can't envision how to initiate a face-to-face meeting.
I'd like to STRONGLY recommend that you check out the communication techniques outlined in Non-Violent Communication. There are fairly straightforward steps that you can learn that will help make communication more compassionate, respectful, and receivable.
Finally, do what you feel you can for the children. They will be better for it, and be reminded that the parenting they see at home is not the only way to raise children. Be aware that this could further enable the drinking problem, however.