Complex sentence: Future Max and Future Liz sacrifice their love in order to make Tess stay in Roswell (in their timeline she left after Max and Liz cemented their relationship), because she was obsessed with Max and she would stay only if Max will give her a chance.
Split into simpler sentences: Future Max and Future Liz sacrifice their love in order to make Tess stay in Roswell (in their timeline she left after Max and Liz cemented their relationship). Since she was obsessed with Max and she would stay only if Max will give her a chance.