I'm thinking of creating a group chat for just my 3 sisters (30's) and I without our parents included (we have a full family one already) because they kept omitting/ not telling us about being unwell
Why do you think your parents would keep hiding that information form you or your sisters?
They are so notoriously private. They would talk among themselves about illnesses, depression, not feel good, financial problems, family conflicts behind our backs.
It sounds like your doing this in revenge. That will just affirm that it's okay to do this in secret. What you should do is call your parents out.
Okay, I see your point and will try talking to them.
Great! Let your parents know that you care about them and you want to be clued in on how they are doing because you love them.