Im comfused, can someone explain the ending for me?
Okay, so, the chapter starts by showing what appears to be a time skip to their happy future life, but then it's heavily implied that it's just the final scenes from main character's manga (or how Kouta Moriyama imagines the larger story behind his manga's final scenes). It's unclear whether that happy ending will actually come to fruition for the siblings, but it at the very least reflects their genuine wish to start a happy family.
On the one hand, there's a very slight implication, perhaps derived from a sort of "Unspoken Plan Guarantee" trope, that their life together will not be so successful. Notably, the happy ending is explicitly contained within the main character's
fictional work. But, on the other hand, as a final chapter, there's also a slight implication that the resolution we are shown in the "story within a story" is being offered as the resolution to the story at large. Along those lines, you could claim that the story within a story is simply a fancy framing device for a canon flash-forward. There's some evidence for this interpretation, especially because characters are addressed by their real names at the beginning of the chapter, which would not be the case in Kouta's manga.
TLDR: intentionally ambiguous ending