I've never heard of any hard and fast 'rule' about siblings, so I think it is just different from party to party. We've been invited to parties with sibs ( when all the kids already know each other) and not. I have a friend with 4 kids, all close in age and I know for a fact that she NEVER assumes sibs are invited, because they all have their own specific groups of friends.
I can also see why they might want parents there, though. At age 4, a lot of kids are not yet ready to party alone, even if your son is. My son has attended a couple of parties where the mom told me that some parents were staying, but my son was welcome to stay alone, because she knew he would be well-behaved and not get upset or feel shy, etc. It's better to err on the side of caution and ask parents to stay, unless you know the child really well and know they will have fun and behave without mom there.
I think its awful to say that the party mom has 'lazy mom' syndrome!! Who knows what that family's personal circumstances, financial constraints, etc. are??? Not everyone can invite EVERY kid to parties, and certainly not all the kids in the neighbor hood. That does NOT make a mom lazy or selfish- sometimes that's just the way it is. If they can only have a certain number of kids, does that mean her child doesn't deserve a party at all??
I work full time and we have had full-on b-day parties with everyone in the families there, and just small movie parties with 4 friends. Each mom or family has to do what is best for their kid to have a good birthday- why be snotty and judgmental and hold that against them???
You didn't say how many sibs he has or how old. If your other child is a baby, then I would say just bring him. But if the sibs are older or you have 3 kids, then take them to grandma's or a sitters for the duration of the party.
Another option would be if you are good friends with another mom whose child is going, ask if she would be willing to chaperon your son at the party as well? (i would only do that if you and the other mom are good friends and she has watched your son before and vice-versa. Also, you would then owe her one, but it might be worth your while!)
I think the party mom is just trying to make sure there are no unsupervised melt-downs and that the party is fun for the kids and goes smoothly. Not allowing sibs is totally reasonable- some people might have a lot of kids, for example, and these days a lot of parties are on a tighter budget too. Also, as your son gets older, he is not going to want to always be tagging around with his sibs anyway! Just be a gracious guest and respect the party hostesses' wishes.