。
She'll probably have to cook for everyone.
Literal
[topic-は] everyone [for the sake of-のために] cooking [object-を] do-must probably.
~なければならない is the canonical formal/written 'must' — literally 'if not [doing X], it does not become.' In casual speech the contractions ~なきゃ(いけない), ~なくちゃ are far more common. ~ために links the action to its beneficiary: みんなのために ('for everyone's sake/benefit'). でしょう adds speculative softening — 'probably will have to.' The sentence touches on a cultural undercurrent: cooking responsibility in Japanese households still falls disproportionately on women, especially in extended-family or multi-generational settings — though attitudes are slowly shifting.