If the plan depends on someone else changing, it isn’t a plan. It’s a hope.

You can only reliably schedule your own behaviour. Something that needs your brother to be organised, your manager to be fair or your friend to stop being late is resting on what you do not control, and it fails without anybody being to blame. Hoping is allowed; it just should not be load-bearing. The structural parts have to be made of your own actions.

Underline every part of your current plan that requires somebody else to be different. Replace one with something you can do alone, even if it is clumsier. Where you cannot, name the backup now rather than mid-collapse.