Often, it's shown through Peter's internal monologues. He might think about how Morgan gets to have a normal family life with Tony while he has to hide his identity and fight crime. For example, he could be seeing Morgan have a simple dinner with Tony and feel a pang of jealousy because he rarely gets such a normal moment with Tony.
His relationship with Morgan herself could also be affected. He might not be as kind to her as he should be. Instead of seeing her as a little girl who just happens to be in a situation where she gets a lot of attention, he could start to resent her, which would make their interactions awkward and uncomfortable.
Peter could be a character with some heroic or special qualities. He might be a young man with a big heart. Morgan could be a young girl, perhaps a student at the school. They could have a special bond, like an older brother - like figure (Peter) and a younger sister - like figure (Morgan).
Well, in these fanfictions, Sasuke's jealousy can be quite intense. It can be seen when he gets angry or distant when another male character is around Sakura. For example, if a male teammate gets too friendly with Sakura, Sasuke may start to give him the cold shoulder. It's a way to show that Sasuke has deep feelings for Sakura and doesn't like the idea of someone else getting close to her.
Garu's jealousy in Pucca fanfiction is often shown through his actions. He may become more withdrawn or brooding when he sees Pucca with another character. For instance, he might sulk in a corner or train more vigorously as a way to distract himself from his jealous feelings.
It can be shown through his actions too. Maybe Merlin is a bit more snappy or short - tempered with Arthur when he's feeling jealous. Like when Arthur asks Merlin to do a task, Merlin might do it with a bit of an attitude instead of his usual eagerness, because he's jealous of something Arthur has or has done.
Sakura's jealousy in these fanfictions can also be expressed through her words. She could make snide remarks about Hinata's relationship with Sasuke. Saying things like 'I don't see what he sees in you' or something along those lines. It adds a lot of tension to the story.
Often, his jealousy is manifested in his actions towards James. He might try to outdo James in academic or magical feats, all because he wants to prove that he is better than James, especially in Lily's eyes. And when it comes to Lily, he may be secretly longing for her attention while trying to hide his feelings because he knows she is with James.
Sakura's jealousy of Hinata can be portrayed in a more subtle way too. In some fanfictions, she might distance herself from Naruto when she sees Hinata getting closer to him. Instead of showing her jealousy outwardly, she'll withdraw and become more focused on her training or other friendships. But it's clear to the readers that she's jealous because of the way she'll steal glances at Naruto and Hinata when they're together, and the sad look in her eyes when she does so.
In many fanfictions, Peter might feel replaced by Morgan because Morgan is Tony Stark's biological child. However, it's important to note that Peter also had a special bond with Tony. Maybe the new focus on Morgan in the post - Endgame world makes Peter seem sidelined in some stories. But this can also be a chance for character growth for Peter, as he learns to find his own place.
One great 'Morgan and Peter Parker fanfiction' is 'The Unlikely Bond'. It beautifully explores their relationship in a post - Endgame world. The story focuses on how Morgan, with her Stark - like intelligence, helps Peter with his scientific projects at school. Another good one is 'Morgan's Protector'. In this, Peter takes on the role of protecting Morgan from threats that emerge due to her connection to Stark Industries. There's also 'A New Friendship' which shows how they gradually become friends and support each other through their respective traumas.