The reveal is important because beforehand she had given her daughter up and now her daughter is an adult. Time travel is a big element in the story and it is what makes the reveal possible. Like I said, depending on the flow of the story, I can make the reveal earlier than intended.
It sounds like things are still in flux with your story so I'd concentrate on getting it down onto paper and you'll figure it out as you go. I think once you get further into it you will know when the reveal is important and also whether or not you will need a sequel.
Yeah. I kind of have writers block at the moment so going forward in writing it is hard at the moment.
Don't wait for a sequel you might never get to reveal an aspect of the plot that you feel is important to the story.
Personally I tend to make "Arcs" based on introducing characters and then events, so twists usually happens at the scene separating those. With that said I like to fill my stories with small twists which aren't shadowed much but are present in number, and along with a huge amount of foreshadowing and smaller misleading decoys create a forest before the huge tree which is my final twist(s). So what I often do is reveal part of the twist which readers who stop to think can find out, yet mixing it with incomplete or sometime carefully stated hints that still leave the revelation as both coherent and something which readers can theorically find out by themselves. I put it this way: It was so painfully obvious. Doing things that way however require some advanced planning and often trashing or rewriting parts of the story, so this may not necessarily be anyone's cup of tea. With that in mind, I also have to admit that coming up with ways toward further insanity and mind-screws for both the characters and any potential reader while avoiding crackly situations is one of the great things which drive me to write.
I figured out that I can introduce her(the daughter) earlier in the story. If I do that, then would it be appropriate to reveal that she's the MC's daughter at the end?
It's your story and these are decisions you have to make on your own. No one else can tell you how to write your story because it's yours! Try it and see if you like it. If not you can always take it apart and redo it. I pretty much tore my current project down the ground and restarted it. It can be done and I'm much happier with it now. Sometimes that happens and it's alright. So be brave and try things!