Complex sentence: Se-Joon planned to make his sister crazy about him after she had her heart transplant because he felt lonely and missed his sister a lot, so he spread rumors about her to make her rely only on him.
Split into simpler sentences: Se-Joon planned to make his sister crazy about him after she had her heart transplant because he felt lonely and missed his sister a lot. Due to that he spread rumors about her to make her rely soely on him.