"Ma mère et lui sont frère et sœur."
There was a discussion about this sentence from two years ago, but my particular question wasn't answered there and I'm afraid my comment in the thread will get no attention, so I've opened a new discussion. I hope that's OK.
The task was to translate "My mother and he are siblings" with a word bank. In my answer, I chose the following word order: "Ma mère et lui sont sœur et frère." It was counted wrong. Is there a reason or is that an error? After all, since the subject phrase is "Ma mère et lui," wouldn't the subject complement phrase be in the same order ("sœur et frère")? Or is there some idiomatic rule that the masculine term comes first?
(Btw, if it was an error, I saved a screen shot. I was on the mobile app, so there was no link to report an error.)