Ok, I don't know WHY, but.... 1. In the US, we would only say, " This is my mom and dad." "This is my brother and sister." These are shorter sentences of "This is my mom and this is my dad." "This is my brother and this is my sister." 2. The only way we might say "these" would be something like, "These two standing over here are my mom and dad." "These two silly children are my brother and sister." I am not an English teacher, so i won't even try to give you specifics of English rules. English rules may differ from the way we speak, but we would only speak the above in the US. Hope this is helpful. :)

"These are my brother and sister" It's correct because it's a plural form.