If I ignore the step sibling thing, which is hard in itself, Jackson was never over April. I mean end of season 14. we see Jackson begging God to not take her from him, bargaining he would do anything if he wouldn’t take April. And through that episode, Maggie clearly has no real understanding of the history between April and Jackson, she just refers to her as the mother of Jackson’s child, all the while Meredith keeps giving Maggie and later Jackson looks because she knows April is not just Harriot’s mother to Jackson.
Then Jackson is never really in a relationship WITH Maggie. We see this almost immediately in season 15. Jackson comes home, Maggie is there, she is making a cup of tea and Jackson sees this and while Jackson is talking with Maggie, he automatically opens a bottle of wine and pours 2 glasses. Maggie turns and puts her glass of tea down next to the wine glass and we see Jackson staring at the 2nd glass of wine he poured. Then we have Meredith confront Jackson at work, warn him not to cheat on her sister and clarify, she doesn’t care who it is he would cheat with, so Mer also knew he wasn’t over April. Its after this that Jackson took off for the woods. Even though he said he went to the woods to grieve when he returns, he brings on the boy who looks like an angel who had his wings cut off intent on doing what no other doctor could do, even goes so far to point out cancerous cells found in the boy, he was just like the boy April brought back from Jordan who knowing it was impossible to fix his hands (according to Riggs) knew and told the boy the minute she met him, Jackson was going to give him back his hands. So after April leaves and Jackson goes into the woods, he brings this boy, and he is sitting next to Maggie and gets a revelation on how to fix this boy and when he is done, tries to act like they did this as a team, just like he and April did with the boy from Jordan and Maggie is all um ok??? Sure?… The whole Maggie/Jackson relationship was Jackson trying to force Maggie into the hole April left and whether you like Maggie or not, she deserved to be with a guy who wanted Maggie for Maggie not because he thought she could be a substitute for someone else. So I’m glad later when he is with Vic, Maggie still calls him out on the fact he has never mourned losing April and was now doing the same thing with Vic and then we see the wine thing again later the day Jo is going to come over to have sex the first time. Same set up, she comes in already drinking, Jackson has set out his wine/two glasses, fruit/cheese tray etc… And we see this also acknowledged in the Jackson/April episode. Except this time, Jackson has made himself at home, he has made a fruit plate, has out a bottle of wine and 2 glasses. When April walks into the kitchen he pours her her glass of wine and she naturally picks up and takes a drink then goes to the fridge and gets what I assume is cheese out to finish the fruit plate.