be up to (one)
1. To be something that one is responsible for or obligated to do. But it's not up to you to discipline my children! We live in this community, so it's up to us to improve it.
2. To be one's choice or decision. It's up to you—what would you rather do this weekend?
be up to somebody
1 be somebody’s right to decide: Shall we have an Indian or a Chinese meal? It’s up to you.The decision’s not up to me.
2 be somebody’s responsibility or duty: It’s up to us to help people in need.