There are contexts in which this would be just fine in (U.S.) English. "Who is it? It is our mom[/mum]." As I recall, some linguists call this identificational (or narrow-focus) articulation, which is used in specific communicative contexts/situations to identify someone or something, answering questions such as "Who/what is it?" (for example, when taking or answering a phone call). The above utterance is another variation of identificational articulation, using a demonstrative pronoun. Depending on the context, it often identifies who "this" is—in this case, "our mom[/mum]". It can also be used, however, to identify who "our mother" is: "This is" (or, more explicitly, "This is our mother"), answering either implicit or explicit questions/assumptions like "Who is your mother?" (for example, when looking at old family pictures/photographs).