Which one of the following is correct?

1. The person we met at the mall yesterday is my cousin.
2. The person we met at the mall yesterday was my cousin.

I remember learning at the school that we're supposed to use present forms in reported speech only when the reported action is a habitual action or a universal truth. Does it apply to relationships also? For example, look at the following sentences:

3. I told her that she is my sister.
4. I told her that she was my sister.

Please clarify. Thank you.

(As I was about to post this, I got another doubt. In my explanation above, should it be "learning at the school", "learning in the school", "learning at school" or "learning in school"? Thanks again.)
Add to your list of 'habitual action' and 'universal truth' the concept of 'still true at the moment of reporting'. The person remains a cousin or sister of the speaker at the time of the report; either form of both sentences is OK.

'Learning at/in school'.
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Thanks a ton, Mr Micawber for the almost instant response. So nice of you! Digressing a bit from this post's topic, can I ask you one more question? Could you please tell me when and when not to omit "the" before nouns?
I presume you mean no indefinite article 'a' either, Elviajero? That leaves us with:

(1) unspecified plurals: 'I love cats';
(2) nouns modified by a possessive, demonstrative, quantifier, etc: 'some/my/this/those cat/cats.

They are likely to be some other cases that I have forgotten.
Thank you again. You have been very helpful. When you find time, please add anything else you think you've missed here. I'll appreciate it much!
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California Jim has kindly reminded me that I didn't address the other half of your question: when not to omit 'the'.

Well, since nothing else significant crosses my mind at the moment, I suggest that 'the' should always be used (aside from the exceptions mentioned in my 1 & 2 above) every time you speak of a specific individual or individuals ('the cat/cats I saw on the street is/are scrawny') or of an individual or individuals previously mentioned in the context ('I bought a cat/some cats'. Really?-- Why?. The cat/cats was/were cheap and I was hungry.')

There are likely further cases which another member will surely address.