First of all, I am so sorry for your loss, and I understand your pain (almost)
I am in almost exact situation with my father with only two differences...1.) he is still alive and 2.) He lives a mile and a half away from me in a small town of 30k people. I, too, have written letter after letter, to no avail. If we bump into each other in public (him, my stepmother or my half brother) they will literally turn and walk the other way. I have 4 children, ages 11 and under, that they have never met. I wish I could tell you why...I wish they *would* tell me why...I honestly have no clue as to what I did. (This has been going on since I was 18 and moved out of my dad's house in a huff because, I thought, I was an adult now and could do whatever I wanted...Yes, I was a jerk, but that was also 15 years ago. Just to clarify, I have never killed anyone of course, nor have I stolen or anything else...the worst trouble I have gotten in to was a speeding ticket.) The last time I contacted my father was about 4 years ago, and he wrote me back demanding to never contact him again (still, with no reason why.) 18 months later, I learned from a cousin that he was facing colon cancer. I desperately wanted to reach out, but I still haven't...not to him. (I found my brother on facebook not long ago and made another attempt, and still have yet to receive a response.)
Many people have asked "He is basically in your neighborhood...why don't you just go to his house?" My response is...he knows where I am at and how to get a hold of me...I am not going to deny the request of an old, sick man.
Anyway, so being in almost the same make and model of a boat that you are in, I wanted to reach out to you. And let you know you are not alone. Advice? You can send a card or flowers, but don't expect anything back. do it for closure. Or, find out where he is buried and visit his gravesite and have your closure there. I agree with others, your dad was a grown man. Even if she is the Wicked Witch of the West, fact of the matter is that, had he wanted contact during those 15 years, he would have found a way to find you. It's hard to hear and accept, but it is true. We don't want to beleive that daddy's treat their daughters this way, they aren't supposed to. It is easy to try and convince yourself that this is the other wife's fault, because real daddy's would never do that.
But some do, darlin...and I am so sorry than you (and I and however many others) were chosen to be the daughter's of dad's like that.
Take care, and I hope you find some peace. big hugs.