Step Families Support Group
Traditionally, a stepfamily is the family one acquires when a parent enters a new marriage, whether the parent was widowed or divorced. This community is focused on many of the relationships and issues that may arise within stepfamilies where you may find answers from some members and you may help other members with their questions.
I am sure everything will be okay. I also don't think there is something wrong with you. Blending families is a very difficult thing. Things will get better just hang in there.
Has there been any reason for the relationship to be strained? Not that that has really any reason for your SM to behave in such a matter, she is the adult in the relationship.
Maybe have a nice talk with your dad about wanting a better more loving relationship with your SM and how to go about doing that. I hope it all works out young lady!!
http://www.dailystrength.org/people/4097047/journal/10691627 for details.
Not knowing anything about you or your father's fiance, I can only encourage you to work on this relationship now rather than later. I'm not sure how long she has been a part of your lives, but it can only help to improve your given situation to find common ground and build on it. Hang in there--you may find your efforts will result in a healthy and loving relationship for all of you.
Best of luck!