Is a comma important?
Write assignment 1.10:
Tell me who you have in your family using the yo form of the verb tener starting with "En mi familia..."
My answer: En mi familia tengo uno padre,una madre y una hermana.
Computer answer: En mi familia, tengo una madre, un padre, dos hermanos y tres hermanas.
Is that comma important?
The comma is used here because the prepositional phrase is being used parenthetically. You could omit it entirely and still have a complete, meaningful sentence.
You cannot have a mother, father, and sister in any other family than you own so the qualifier (in my family) is totally superfluous. It's only there to emphasize that you are discussing your family.
If the prepositional phrase were needed for the sentence to have meaning then you would not offset it with a comma.
And to separate the elements (except the last) of a series e.g. "un padre, una madre y una hermana"
Note that in English, a comma before "and" in a series is optional but in Spanish, it shouldn't be there.
No sé la regla en español. Pero en ingles, the rule I use is that you use a comma to set off an introductory phrase if it's 3 words or longer (like "in my family,"). But 3 words feels a bit short in some cases; some will only use it when it's a 4-word+ phrase.
Hope that helps!