Perhaps she's discussed bigger/smaller siblings with friends at school? That might be why the change suddenly, and how she has figured it out... her friends/playmates maybe don't like their siblings?
My kids always (well, almost all the time) got along quite well and played nicely together. Boy and girl, 3 years age difference. I can't really say why. Maybe some kids just click better. Maybe it's more of a function of their personalities. Maybe it's more driven by how our family operates/ed as a whole. I don't have any way to know which parts or how much of which parts played the bigger role.
They are teens now. Son is a senior in high school, daughter is a freshman. She rides with him to school in the mornings this year. These days they tend to have their own friends and do their own thing. But they mostly get along when they are together still.
She has always wanted to do whatever he could do. And she was very capable. So the age gap didn't make as large of an issue as you might imagine. Except for things like roller coaster/rides as the amusement park. There was a brief period when she wasn't tall enough yet to ride all the things he could.
The only thing I distinctly recall, with any specificity, was one day when she was about 3 weeks old and it was just the 3 of us at home. I was feeding her, and he wanted to show me something (or something). I told him that I would be happy to see it, but first I needed to finish feeding his sister. It was the first time he expressed any anger about her disrupting his world. Because really, except for that sort of thing, she didn't disrupt his world.
I gave him this whole spiel about how we're all family and we all love and look out for each other. He's the only brother she has. She's the only sister he has. And while he now needed to share time with his dad and myself, she was born into a world with a brother, and would never have time with us that didn't involve sharing with him. He had 3 years where we took care of all of his needs without needing to share. Now, she needed taking care of, and she would be, but she'd still be sharing.
When he was a baby, we fed him, changed him, rocked him, read to him, etc. Just him. She was getting fed, while I was reading him a book. Or getting changed so I could carry her into his room and look at what he had built. I rocked her and talked with him at the same time.
So, even though the ways we spent time together were a little different and not always without sharing, it was all she would know.
Now, did that have any impact on him and how he viewed his sister and their relationship going forward? I have no idea. But he never complained about her. He was patient and loving. He showed her how to hold toys, what to do with them. He loved to make her laugh and dress her up like superheroes. When she was a toddler he'd go in her room with her and put every hair bow she owned in her hair and then bring her out to me to show her off. He loved having her around.
But I can't say what we *did* to cause it. Or even that we did *cause* it. He was never allowed to speak ill of her or to her. That was corrected. And vice versa. She wasn't allowed to hit/pinch/poke him, either. He was taught that boys look out for and protect girls. She was taught to respect persons, and that it wasn't ok to take advantage of her being a girl to bully a boy.
Without knowing how you interact with your kids (both together and individually) it's impossible to make any kind of suggestion about what you might try to do differently. Neither of our kids were in preschool before age 4, so I'm not sure what might have been different if they had. Perhaps your daughter feels "sent away" so that you have alone time with the baby? (and I'm not suggesting that is a bad idea, only how she might perceive it.)
Just try to include her as much as you can, without forcing her to "work" to help. Keep things fun and let her help as much as she is interested. Let her feel appreciated and like she is truly helpful.
Good luck.