What did you say when your DB mentioned it? You say you assumed they were going with friends or on their own, did you not ask them anything about it?
I think you need to be careful what you say to the group, depending on what the conversation with your DB was like, I mean this kindly, but you're bordering on playing the victim here by saying they didn't specifically 'invite' you and your feeling are hurt by them excluding you.. you honestly don't 'invite' people to come with you to these things, you mention it and if someone shows an interest you make plans to meet up there or go together.
This is how it would normally go:
DB: we're going to 'event' in a few weeks.
YOU: sounds great, who's playing?
DB 'band name' are headlining this year.
YOU: I've never been to 'event' but always thought about going.
DB: Why don't you come? I've already asked sibling X if they want to go, if they say yes, we could all make a weekend of it!
VS
DB: we're going to 'event' in a few weeks.
YOU: that's nice, hope you have fun, changes subject.
DB then has the same conversation with your sibling in which they express wanting to go so they make plans, they may have asked about you, but DB would then say "I've already mentioned it but they didn't seem that interested".
Only you know how the conversation went OP, if they don't have form for purposely excluding you It really could just be a simple miscommunication/misunderstanding.
I'd say something along the lines of, "your pics from 'event' looks great, I wish I knew you were all going as I'd have liked to have come along too", no need to accuse anyone of anything but it may make them realise you're a little unhappy about being overlooked.