Another option for the main villain could be a power - hungry nobleman. He wants to marry Beauty's sister for her family's wealth and status. When he discovers that she is involved with the Beast's situation, he tries to stop her from helping the Beast break the curse. He might spread false rumors about the Beast in the village to turn the people against the idea of the curse being broken and the Beast being redeemed.
A jealous witch who was the one who originally cast the curse on the Beast could be the main villain. She might see the little sister's presence as a threat to her plan of keeping the Beast trapped forever. Maybe she tries to trick the little sister into making things worse for the Beast.
The main villain could be a greedy hunter from the village. He hears about the Beast's castle being full of treasures and decides to use the little sister to get to the castle. He could pretend to befriend the little sister but actually has bad intentions. He might try to steal the magic items in the castle that are crucial for breaking the curse and also endanger the lives of the Beast and those in the castle.
It could be Belle's sister herself. If she's envious of Belle, she might turn into a villain. She could try to keep Belle away from the Beast and the castle forever, perhaps by locking Belle up or convincing the villagers to keep Belle away. This would make her the main source of conflict in the story.
It could be a relative of Gaston. Maybe a cousin or a younger brother who has always held a grudge against Belle for rejecting Gaston and causing his death. This character could be plotting to ruin Belle's family and take over the castle. He might try to turn the villagers against Belle's daughter by spreading false rumors about her family's dark secrets.
It could be an envious witch. She was once in love with the prince (now the Beast) but he rejected her. So she cursed him and now she wants to make sure he never finds true love with the Dark Beauty.
One idea could be that the little sister has a special gift that helps break the curse in a different way. For example, she might be able to communicate with the enchanted objects in a more profound way and they guide her to find a solution. Another idea is that she gets lost in the forest on her own adventure and stumbles upon the Beast's castle before her sister does. She could form an unlikely friendship with some of the castle's inhabitants which then sets the stage for when her sister arrives.
It could be Trigon. Since Raven has a connection to him, he might see the baby as a way to get revenge on Raven or as a powerful tool for his evil plans. He could try to corrupt the baby's powers for his own use.
The main villain could be a time - traveling version of one of his regular foes, like Slade. Maybe Slade from the future has developed a way to manipulate time and he's going back in time to stop Beast Boy from ever becoming a Titan or to change events in his past that will make him easier to defeat. Another option could be an ancient evil that was sealed away in the past and Beast Boy's time travel accidentally releases it. It then tries to take over the present by changing key events in history. And it could also be a future version of Beast Boy himself who has been corrupted by power and is trying to change the past to make himself even more powerful.
It could be Shredder as always. He might see the sister as a new threat or a way to get to the turtles. He could try to kidnap her to lure the turtles into a trap.
Krang from Dimension X could also be a great villain. He might be interested in the human sister because she has some unique knowledge or ability that he wants to use for his own evil plans in taking over Earth. He could send his minions to capture her and the Turtles would have to journey to Dimension X to save her.
Perhaps Bellatrix Lestrange had a secret accomplice who was also interested in the Potter family. After Bellatrix's death, this accomplice decides to hunt down the missing sister to finish what Bellatrix started - destroying the Potter family once and for all.
Slade could be a great main villain. His combat skills and strategic mind would pose a huge challenge for Beast Boy. Beast Boy would have to constantly outwit and outmaneuver Slade, which would really bring out his badass side as he tries to protect his friends and the city from Slade's evil plans.