Unless stated, asking to bring a sibling is being a complete CF.
You can say, sorry Olivia can't attend as I don't have anyone to watch Raphael, and leave it up to the parent to offer. Even asking is being cheeky as it puts the parent in an awkward position.
Even if the venue is something like softplay, as it's not a family outing for you, it's a party for the child having a birthday. It's rude.
Stop doing it! AIBU?