any english native can help me with this?

In my opinion, it can be either:

There is a brother and a sister.

or

There are a brother and sister.


The first is more likely to be used in my region (Texas/United States).

DrewDad is correct in stating that both expressions are acceptable. However, there is a subtle difference in the meaning, depending on which one is used. In
the sentence "There is a brother and sister," the pair are being referred to as a unit, almost inseparable. In other words, 'brother' and 'sister', put together make up this duo. In the sentence "There are a brother and sister," the implication is that the two are quite distinct and separate persons.

It's very subtle but the difference is there. As far as proper grammar, you can use either expresion.

Thanks so much, and I wonder: since we can say, "there is a knife and fork", why can't we say "there is a brother and sister"?

very clear explanation. Thank you so much!