I don't think that a single decision made by the parents when the child is 4 is going to mean they are a spoilt adult.
It is quite amusing that people think not being a bridesmaid when they are 4 won't have an impact in one direction, but will in the other.
Bottom line is, we don't know the children. If the 4 year old is one who is likely to be upset, they have not only got to process the facts when they are told, but live them through the wedding day. When they see their sister being bridesmaid.
Maybe she'll get upset at the service, when the parents will be expected to keep her quiet.
Maybe she'll get upset when the older sister is having extra photos taken and having a fuss made of her because she's a bridesmaid.
It's easy to say, "well she just needs to be told" but weddings are stressful, long days with young children at the best of times, when you know a lot of money has been spent, and everyone has ridiculously high expectations of everyone else and their conduct/behaviour.
OP already has a thread about her mum's friend (who she barely knows) stropping off about not being invited.
This is not a stress free situation.
The parents of the child probably just don't want to be part of any problem on the day. Don't want to spend time and money going to a wedding where they're going to have to manage the disappointment and upset of a 4 year old who is already going to be overwhelmed by everything that is going on around them. Don't want the job of keeping them quiet during the service, and the meal and all the boring bits when the photos are taken and the transition between service and reception, which are all pretty tedious for anyone there who isn't actually getting married or close family, and probably just thought, "do you know what, for an easy life..."
Rather than just saying, "no", they told you why. We don't know what OP's response was but it prompted the parents to say that it was unreasonable to ask one and not the other.
It was all ended and she asked someone else.
No. Problem.