@GracieGirl,
Gracie, you were predicting gloom and doom about having to stay with your grandparents, but you wound up having a good time and feeling closer to them. It really wasn't a bad experience at all. Things you worry about beforehand don't always turn out to be bad, and, in fact, you did learn, and enjoy, and discover, all sorts of positive things that you hadn't imagined or anticipated. And, on top of that, you gained a lot of new clothes.
So, you can't always predict how things will turn out...
What's going on in your family right now is disruptive for you, and it's unfamiliar, and you aren't sure where your life is headed in terms of a move, or a new home, or a new school. And you don't feel in control of any of it. That sucks. It's anxiety provoking and very unsettling. I can understand why you feel upset.
Life is always full of surprises and changes, some of which seem immediately pleasant and wonderful, and others just disrupt our sense of security by altering things we've taken for granted in ways that we're not happy about. That's life. You can't stop change from happening. And part of growing-up, in the sense of maturing, is learning to roll with the punches, learning how to accept and adapt, and learning (hopefully) how to deal with the emotions that change generates so we don't let feelings, like anger, make a difficult situation worse, or snafu possibilities for future happiness.
You're entitled to feel angry, and upset, and confused, and even a little bit scared, right now--your familiar world has just been altered and you don't know what's coming next and how it will affect you. And Katie may be a handy scapegoat for those feelings because it seems easy to blame her, and your dad, for doing things that wind up affecting you, and having consequences in your life--like being in love, having a baby, and forming a new, blended family unit. That sort of blaming and scapegoating, while understandable, won't help you one bit. It can only wind up damaging your relationships and it really won't help you deal with whatever changes are going on, and might be looming in the future.
Your challenge right now is to figure out a better way of handling your feelings rather than acting on them by being bratty, or sullen, or argumentative, or yelling. I know that's easier said than done, particularly at 14--I was once there too. But you're much better off talking things out, simply talking about your anger and confusion, with your dad and Katie, without accusing or blaming them--just express what you are feeling, so they can understand and appreciate what you are going through. Don't try to make them feel guilty, don't try to manipulate them to bend to your wishes, just tell them how you are feeling and what you are concerned about. Just being understood is important, very important, so help them to understand you.
And, the other thing you should do is just take these changes one step at a time, as they happen. Don't worry now about going to a new school, it's not something that's at all definite, so why worry in advance. Just deal with what's going on right now--like a lot of new people living in your house, and suddenly having an adult female in your house--and your dad sharing his time and attention with those people. Everyone in your house is going through a period of adjustment, including Katie and her children. BTW, how many children does she have? Do you get along with them?
A new family unit is forming and emerging for you, and it's going to include Katie, and soon, a new baby brother or sister for you. Things aren't the same as before, but, I hope that, as time goes on, and the unfamiliar becomes more comfortable, and part of the fabric of your life, that you will find your newly expanded family enriching your life and providing you with even more love and support than you had before. Adding good people to your life is a lot better, and easier, than losing them. Things may be awkward, and confusing, and unsettled, now, but, hang in there, you may be pleasantly surprised by how things turn out. Think positive.