Be flexible regarding boundaries, yet remain consistent and firm when setting limits. You need to be flexible because your family member’s comfort level, needs and behaviour might change in different situations (e.g., at work versus at home), at different times (e.g., on the weekend, during the night, during vacations), with different demands (e.g., a request to borrow a car, a spontaneous visit) and with different people. As a result, your boundaries may change.
It is also important to be consistent and firm when setting a specific limit with a specific person in a specific situation. You must clearly outline the consequences of breaking that limit. The reality is that setting limits is difficult and may be risky. You will need to figure out for yourself what risks you are willing to tolerate.