逆權侵佔(是否只限業主與租客)

The answer is no at all.

In order for an adverse possession to be succeed, the person must hostilely possess the property in question.

Since your mother has 50%, the hostile issue will not stand (as she has the right). For you, since your mother allows you to stay, the hostile issue will not stand as well.

Your mother can file a lawsuit against your relative for the past expense. But your mother has no chance winning an adverse possession case.

法律改革委員會在2012年 引入業權註冊制度及「十加二年期」方案，即霸地者連續佔地十年後，有權註冊業權，但同時須通知業主，如業主在兩年內不反對或理據不足，霸地者才可申請註冊為業主，結果交由審裁官裁決。你要進行逆權侵佔的話, 需以上述制度為依歸, 最好向律師徵詢有關意見.