Reciprocal pronouns in English

There are the following reciprocal pronouns in English:

Example: Your brother hates his sister and she hates him. We can say that they hate one another.

We help each other a lot.
She and her husband understand each other.
They send each other text messages from time to time.

There are two reciprocal pronouns: each other, one another.

The reciprocal pronouns are also used in the genitive case:
The friends borrowed each other's books.
They lost each other's notes.

If we join two sentences of the type:
Tom married Barbara. and Barbara married Tom.
we obtain:
Tom and Barbara married each other.

each other is usually used when two antecedents are involved.
one another usually occurs with more than two antecedents:

Tom and Mary cheated each other at cards.
The four friends cheated one another at cards.