It’s not really a social club. It’s an activity.
How old is the young sibling? I’m assuming not yet in K? For a child that young, yeah, you can’t just expect the den leader to entertain the younger sibling while also leading a meeting. As your younger child gets older, they will probably also join scouts, and then they are usually welcome to participate in the activities. Our den leader has a Webelo and a Wolf. The Wolf comes to the Webelo meetings, and it’s fine.
As for missing meetings…it’s OK to miss here and there. We usually have at least one missing from each den meeting, and probably several from the pack meetings. You can make up the activities for the den meeting at home, or the den leaders sometimes pull scouts aside at the next meeting to wrap up advancement requirements.
Pack meetings are where awards are handed out, and flag ceremonies are done, and where they learn the songs and skits. I feel like those are easier to miss, but that’s also where a lot of information is given to parents, or sign-ups happen. A lot of this is done by email/online though.
There are weekend events too: scouting for food, camp outs, hikes, popcorn sales etc. You need to do a service activity every year, which usually means one of the two Scouting for Food events. They ask that you sell popcorn, but it’s not a hard requirement. A lot of badge or advancement requirements are knocked out at camp outs.
But in cub scouts, you really get out what you put in. My primary motivation was finding some friends for my kid, and giving him the chance to learn outdoor skills. If we don’t go to the meetings, we’re not really accomplishing those goals.