Over the years I've received many letters like yours -- and one reason I wanted to reprint your letter is because I hope it will encourage others not to make the same mistake your father did.
No doubt your father was wrapped up in his own emotions and plans at the time -- but he should have been considerate of yours as well. By not discussing his plans with you, he not only ignored your feelings but also set the stage for much resentment and hurt later on. The Bible says, "Each of you should look not only to your own interests, but also to the interests of others" (Philippians 2:4).
However, you can't change the past -- and you must not let it poison the future either. No doubt you and your sisters cherish the memory of your mother, and in your minds no one can ever replace her. That's perfectly understandable -- and it's not wrong. But be grateful that your father once again has someone with whom he can share his life -- someone who loves him and can help take away the loneliness every person faces who loses their spouse.
Reach out to your stepmother; she needs your friendship as much as you need hers. Don't dredge up the past with your father, but make an effort to get acquainted with her and accept her. Thanksgiving would be a good time to start.