S'entremêler means 'to intertwine' or 'to get mixed up'. It's a reflexive verb that beautifully captures the idea of things becoming entangled.
In the song, Angèle sings, "Mais pourquoi s'entremêler entre lui et moi?" (But why get mixed up between him and me?). She uses this word to express the frustration of jealousy creating confusion and interference in her relationship, highlighting how an outside presence can complicate things.
Belgian artist Angèle has a real talent for making songs about difficult feelings sound incredibly fun, and "Jalousie" is a perfect example. She sings directly to the feeling of jealousy itself, treating it like an annoying person who keeps interfering in her relationship. She complains that for such an ugly feeling, jealousy has a name that's "bien trop joli" [way too pretty].
She shows how this feeling gets into her head over small things, like seeing a photo and asking, "Mais c'est qui cette fille sur la photo?" [But who is this girl in the photo?]. She personifies jealousy as a voice whispering paranoid thoughts, telling her the other girl is beautiful and wants her boyfriend. The song perfectly describes how doubt "campe dans nos têtes" [camps in our heads] and makes everything feel much worse than it probably is.