I am a special events coordinator and it is never ok to assume siblings are invited.
Weddings,Birthday celebrations, dinner parties, showers are only for the people who have their names on the envelopes. It is considered very presumptuous to assume you can bring others.
If others are invited, you will include, "siblings welcome". The name and the name of the guest or number of guests also allowed will let you know if you can bring others.
Gift bags, meals, prizes, space, transportation all take careful planning and money.
If you have a problem getting your child to an event because of your other children, you may want to ask the host or hostess if their are others who may be able to give your child a ride.
There may be times your child just cannot attend. Children need to learn they are not invited to every event just because one of their siblings is invited. This is a chance for your child to attend on their own. They need to learn independence. It also will allow you some alone time without a child to concentrate on the other children. This is not a snub. Do not take it personally.