By the way you can make Cassiopea and her brothers hate Sirius , because he abandoned his father mercilessly leaving him with Walburga, the problem was Regulus also wanted to leave but couldn't when his own mother threatened him for going straight to Sirius to force him to be a wizard morph wizard, if it meant hurting the Potters to get to her traitorous son.
Regulus as much as he resented Sirius due to many things, it was his brother who protected him before they both went to Howargts, only for Sirius to change by being in Gryffindor house and due to Albus' manipulations.
So Regulus decided to do one last time for Sirius, even if it meant breaking their relationship forever. And that's what he did, he took the role of heir to Black house and the mark to prevent Sirius from being.
Regulus told his beloved Madeleine, she instead of rejecting him supported him, saying that Sirius didn't deserve to have a good brother like him.
Then Cassiopea and her brothers found out about their uncle's treatment of their father for being a Slytherin at Howargts, another reason they hate Sirius.
And lastly you can make them resent him even more because Sirius works with the person who indirectly killed their parents, that would be Albus himself, when that eccentric old man was telling Voldemort where the location of House Black was.
And one more thing, they haven't thought they can have the parents of the three Black siblings brought in, including Harry's parents to see Cassiopea's life.
And there James and Lily realize how foolish and ignorant they were to trust Albus too much about Harry, and that they were the ones in the wrong, not the Slytherins.
And even Lily and James realized that they had been wrong about Severus, about Albus, and about more people.
James would regret having ignored his parents' advice and his sister's warnings about Albus
