"Her brother is two."
Translation:Son frère a deux ans.
Its odd that your earlier post has no time stamp, I wonder why that is? Now on that earlier post you asked about the ambiguity of the phrase, Her brother is two. I can't imagine any other possibility other than referring to his age, what were you considering?
My original question was ambiguous also, I was not only asking if you can leave out the ans, but also can you use, est, instead of, a?
The first time I saw this question (more than six months ago) I couldn't understand the meaning of "Her brother is two." at all, it did not cross my mind that it could be referring to the boy's age. Maybe because english is not my native language... So I figured it was a nonsensical phrase on purpose.
To your french question, no, you cannot use the verb être (est) instead of avoir (a) in this case. I think you can omit 'ans' if it is very obvious. For example: "Sa soeur a dix ans et son frère a deux." but I'm not completely sure this is considered correct.