How to be friends with your step-children
Though my generation of siblings and cousins seem to (almost always) marry once only, my parents were more inclined toward serial monogamy (I think). My mother has married twice, and lives with a new partner now, my father is twice-married too. In the case of both my parents, I was not expecting to be step-parented. My mother moved in with her (later) husband when I was 16, so I left home. My father married the new lady in his life when I was in my early thirites.
The contrast between the two approaches from my parents could not have been more different. My mother never expected me to acknowledge her new man as someone significant to me. We (my sisters and I) were happy for her - though her man was a domineering kind of character, he treated her like a princess, no doubt manna from heaven for her after some years of emotional neglect. He, for his part, did not expect to be parenting us, and it made life alot easier that this was the case.
My father, later, encountered a very strong-willed woman, with whom he fell in love. I met her on a visit back home, and, initially, liked her. But she insisted on referring to herself in later letters as 'the wicked stepmother', and would try to talk to me rather than let me speak to my father when I phoned. This irked me greatly, particularly as we, Dad and I, needed to work through some difficult issues arising from my childhood. His wife was not there at the time, and I didn't feel it was her business. But she became his gatekeeper, and eventually I just 'lost it' with my Dad - I threw my rage at him, and was never allowed to have contact with him again, unless I agreed to negotiate to do so through his wife.
I though about this all recently when I saw an article in a newspaper about this very subject. There was mention of how difficult relations can be, if the new person (husband or wife) insists on being included in family life.
It is sad what happened. Now my father is in a rest home and I feel loathe to pressure either of my sisters to reveal where it is, although one of the two might relent, the one who is not interested in the gate-keeping behaviour. What do readers think? My father is suffering from the early stages of dementia and seldom comprehends that he is being visited (I have heard he sleeps alot). Should I try to go there on my own terms (i.e. without checking with the gatekeeper)?

How terribly sad. Yes do go visit your father, do not let him go to his final earthly sleep without your telling him you love him,deep in your heart you do,thats what drives you; you will regret it til you die. My relationship with my Mother was not the best (not like your situation) how I wish I had told her I loved her. Its all too late when the last bell has chimed!!