I think a lot of it depends on the relationship you have with the parents. I was often in situations where I could not leave DD2 with someone else and at 5yo, there was no way I would leave DD1 ata HJ party (home party yes, but not a public venue). Some parents knew and understood, even saying with the invite that DD2 was welcome, or if approached were fine with her coming along. Maybe it was due to many of DD's friends also having younger sibling

In return I tend to be ok with younger siblings as I understand it is not always practical. Now DD1 is older and can be left easily, it is no longer an issue, but at DD2 seems to be friends with DD1's friends siblings, she often gets her own invite.