Translation:The two brothers-in-law speak with the old aunt.
I asked that question 3 months ago, I've learned many more things now. In this sentence it really is just a definite article. As far as I know they don't really put definite article -ul on numbers, so when there is a number such as here they use this way to show the definite article.
They sometimes use it to just accent on an adjective, though I am not sure if there is any other reason, in phrases like "calculator cel nou" - "The new computer".
And if it is used alone it means just "the one(s)": "Cei pe care îi iubesc" - "The ones I love"
Disclaimer: I am not a native speaker just studying it, so I don't know how accurate my answer is.
"bătrân" is used for people. "bătrână" is the feminine form of the adjective.
"vechi" is used for objects.
I am not sure if "bătrân" is only for people or for all living things. That is, I am not sure how you would refer to an animal, but I'd guess "bătrân".