The meanings of the two words overlap, but there is still a distinct difference. Please review the words in English. If you just remember the Chinese translation, then they are virtually indistinguishable.

related, relative and relevant 三個「相關的」單詞
BBC Audio: https://bbc.in/2VAftuA

1. be relative to

If something is relative to something else, it changes according to the speed or level of the other thing.
按…的比例；與…相應

The amount of petrol a car uses is relative to its speed.
汽車耗油量和其行駛速度是成比例的。

If something is relative to a particular subject, it is connected with it.
與…相關；有關；涉及

Are these documents relative to the discussion?
這些文件和我們討論的問題有關嗎？

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2. be relevant to

be connected with what is happening or being discussed
有關的；相關聯的；切題的

Education should be relevant to the child’s needs.
教育應和孩子的需要掛鈎。

The two phrases overlap at times, but there is still a distinct difference. “Be relevant to" refers to something being directly relating to the subject or problem being discussed or considered. It could be important to the discussion.

“Be relative to" refers to something that is connected with a particular subject. It also used in comparison with somebody/something else.

Sources:
https://dictionary.cambridge.org/zht/

https://www.ldoceonline.com/dictionary/relative-to-something

https://www.ldoceonline.com/dictionary/relevant

https://www.oxfordlearnersdictionaries.com/definition/english/relative_1#relative_idmg_1