Learn Russian: Привет! Это мой брат.

In Russian, the present tense of the verb "to be" is almost never used. So, instead of saying "This is my brother," you simply say "This my brother" (Это мой брат). You just put the words side by side!

Since брат (brother) is masculine, why do we use the neuter word это instead of the masculine этот?

When you are pointing something out and saying "This is...", you always use the fixed word это, regardless of the gender of the person or thing. You would only use the masculine этот if you were saying "this brother" as a descriptive phrase, like in "this brother is tall."

Would the word for "my" (мой) change if I were introducing my sister?

Yes! Possessive words like "my" have to match the gender of the noun they belong to. Because брат is masculine, we use the masculine мой. If you were introducing your sister (сестра, which is feminine), you would change it to моя: Это моя сестра.

Can I use Привет to say hi to anyone?

Привет is an informal greeting, exactly like "Hi!" in English. You should only use it with friends, family, or people your own age in casual settings. If you are introducing your brother to a boss or an older stranger, you should use the formal "Hello" instead: Здравствуйте.