It might make more sense if OP had more than one sibling and forgot to pluralize, but since that was done more than once, OP could have a sibling that doesn't w

It might make more sense if OP had more than one sibling and forgot to pluralize, but since that was done more than once, OP could have a sibling that doesn't want to be defined as either male or female, hence "sibling" and not "brother" or "sister". (At least that's what I got from watching the TV series, Billions.)