You and your siblings are in a very difficult situation.
I admire the depth of your love and concern for your dad.
Intervening with a family member can feel very necessary.
Yet, you are right -- when and where?
I hope you and your siblings can show a united front of love and concern. Sometimes these expressions by loved ones melt the heart of the one with whom we feel led to intervene.

I wish you all the best as you decide more about exactly what to do to help your father.

Love and prayers,

WC