I think it's got worse as the age parents seem to stay/expect to stay has got older.
When I was little birthday parties over tiny tots, no parents would stay unless by invitation.
When my dc were little, preschool parties people tended to stay, but not older.
Now I've seen people on here commenting that parents stay until much older.
I think if it's a public entry, then absolutely fine to stay with siblings and pay for them, but you don't expect food /party bags etc, really don't even go into the party room with them, because that can put the host in an awkward situation.
If you know the parent well, then it's fine to ask when there's no public entry as long as you ask in a way that allows the answer to be "no" and you also let the host know that there is no expectation that siblings join in/get food etc.
Other than those situations, then no. It's not fair on the invited child either.
I don't remember when my dc were little, people bringing siblings along to a private place and expecting them to join in; they'd normally sit on the side with things to do.
I was at one once, where a family of 5 with very harassed dad came, with youngest the invited one. The older four sat, rather bored, but pretty good on the side with a small number of snacks he'd brought. He explained that Mum was ill, which was why he'd had to bring them.
The birthday child had just started at preschool, and had asked the preschool teachers to hand out the invites to the 9 children who were in the same room. My dc was invited through another connection.
I was chatting to the dad, when the host came over and in the course of the conversation, it turned out that the child had the same name as another in a different room... and this was the wrong child! Poor dad was mortified, whereas me and the host thought it very funny.