Of course you want everyone to love each other. When people come together to create a family, it is normal for the members of this new family to think that everyone should love each other. Biological parents especially want their new spouse to love their children, and want the children to love the stepparent in return. Stepparents think they should love the children, and children are often confused.
The problem is that couples get married because they are in love with each other, usually not because they want an instant family complete with children. And even if they love the children before the marriage that can easily change. Rather delightful children before the wedding vows can become surly, defiant, or worse afterwards for numerous reasons, especially towards the stepparent. There are many reasons for this, but it creates a bind for the stepparent, who may be doing their best to parent these children.
As for the children, I rather relate to them from a personal experience. About a year and a half after my father died, my mother started dating someone she had met at the Senior Center in her area. I was in my thirties when this happened, and I completely understood and appreciated my mother going on with her life. But when I met Bill, a rather decent, elderly gentleman, I couldn’t get over the fact that he seemed to like my mother. And I immediately disliked him for it.
Things are even more complicated with children. This new person moves in and often begins a process where the entire family changes. The stepparent almost always does things differently than the biological parent, and has new authority to make things happen.
If there is another biological parent still around, any fantasies that mom and dad are going to get back together again, die a final death when the marriage vows are complete. And if there is another parent that is threatened by this new marriage, it is possible that they might give messages, even non-verbal ones, that if the children even like, much less love the new stepparent, it will be a betrayal to them.
In this arena of newness and loyalty binds, it is understandable for children to be slow to warm up to the new parent and for stepparents to be slow to love the children.
So, take the pressure off of everyone. Announce in a family meeting that in your family, no one has to love anyone. In fact, no one has to even like anyone either. Both children and stepparents, often sigh with relief when this expectation is taken off. What you do expect, however, and will enforce, is that people treat each other with respect. This is regulating behavior rather than feelings and is much more likely to be met with success.
And just maybe, with time, love will get a chance to happen on its own. Best of luck!