Answer:
My dad, Aunt Maria, Uncle Diego, and my cousin were all there, but Mom couldn't come.
Step-by-step explanation:
If my is used, there is no need to capitalize Dad -- mother does not need a capital 'M' as it is not being used to replace her name. If I said, "I am going to lunch with Mum", it would need a capital letter, but "I am going to lunch with my mum" does not. We should capitalize these words if they are being used as the name of the person.
Capitalize Mom and Dad as a Proper Noun. When you're referring to a specific person, you may be using the proper noun form. In this case, you would capitalize the words "mom" and "dad." ... If it makes sense, then it's a proper noun.
What about Uncle or Aunt?
Normally, a kinship word like “uncle” is capitalized if it appears just before a personal name, as in this version: “At 10, Uncle Bob will arrive by train.”