1. I have Mom, Dad, and a sister.

2. I have a mom, a dad, and a sister.

Which one is grammatical?

3. I have my mother, my father, and my sister.

4. I have my mother, my father, and a sister.

5. I have my mother, father, and sister.

Among the five, which one is grammatical?

#2 would be the best of the group.

Both sentences are grammatically correct. The only difference between the two is the use of capitalization. In sentence 1, "Mom", "Dad", and "sister" are not capitalized, while in sentence 2, they are capitalized. The choice between capitalization or not depends on the writing style or personal preference, but it is generally recommended to capitalize proper nouns like "Mom" and "Dad" when referring to specific individuals.

Both sentences are grammatical, but there is a slight difference in punctuation. Sentence 1 does not include commas before "Mom" and "Dad," whereas Sentence 2 does. The use of commas in Sentence 2 follows the standard convention of separating items in a list.

To determine which punctuation form is appropriate, you need to consider the context and style guide you are following. In general, using commas to separate items in a list is more common in written English, while omitting them can be acceptable in informal contexts or when adhering to certain stylistic choices.