Learn Icelandic: Þetta er stjúpbróðir minn, Jón.

In Icelandic, words for close family members usually do not take the definite article when followed by a possessive pronoun. So while you would say bíllinn minn (my car, literally 'the car my'), for family you just use the bare noun, like stjúpbróðir minn (my stepbrother) or móðir mín (my mother).

Why is þetta (the neuter form) used to introduce Jón, who is a man?

When introducing someone or pointing them out with 'This is...', Icelandic always uses the neuter pronoun þetta regardless of the person's gender. Once they have been introduced, you would use the masculine pronoun hann (he) to talk about Jón in the following sentences.

How does the word stjúpbróðir change if it is the object of the sentence?

It declines exactly like the word bróðir (brother). It is irregular, so in the accusative, dative, and genitive cases, the ending changes to stjúpbróður. For example, if you said 'I see my stepbrother' (accusative case), it would be 'Ég sé stjúpbróður minn'.

How is the word stjúpbróðir built?

The prefix stjúp- means 'step-' in family relationships. You can attach it to other family words to make new ones, such as stjúpmóðir (stepmother), stjúpsystir (stepsister), or stjúpsonur (stepson).