I'll take this slowly...
You don't appear to be able to grasp that what you have got it in your head - the thing that you are so very sure has happened - may not actually have happened. You may have to consider that what you assume is open to interpretation.
All the OP says is:
My parents invited us for Easter lunch. They didn't really give an exact time said around 5-6.
You seem very certain that this means that they would start eating between 5 and 6.
But it's impossible to know that from the post. It may mean when the meal will be, or it may well be when the OP is expected to arrive.
It is perfectly possible to read either from the information given. The fact that various posters have contrasting interpretations of what the OP has posted makes that abundantly clear. You can keep posting your shrugging emojis for as long as you like, but they don't make you correct or the invitation unambiguous.
I maintain it is quite reasonable to interpret "around 5 to 6" as around 5 to around 6, and to quite reasonable to think that the time was the time the OP was expected to be there, rather than the time dinner was served. Much more reasonable, in my view, than interpreting "around 5 to 6" as meaning "we will serve dinner at 5.30." Much more reasonable, in my view, than interpreting arriving at 5 past 6 as being over an hour late. But all of these interpretations are possible - they have all been put forward by people on this thread, after all.
So of course all of this confusion could have been avoided if the OP has asked what the hosts had meant, or if the hosts had been clearer in the first place. And it would be courteous to let them know when she expected to arrive.
But, considering that she didn't, their invitation remains ambiguous, and it remains extremely rude in those circumstances to start eating when she arrived - at the very worst - around ten minutes after it was reasonable to assume she was invited.