A more thorough explanation:

Sobrini are the children of first cousins, also known as second cousins. This term comes from Roman law.

• My mom's cousin's children are my sobrini.
• When my grandparents were alive, we used to have family reunions with all of our sobrini.

These examples show how sobrini are related to first cousins. In the first example, the speaker's mom's cousin's children are their sobrini. In the second example, the speaker's family has reunions with all of their sobrini, who are likely the children of their first cousins.