I really feel like an explanation would be expected in this situation. I could understand this behavior if they were estranged or had some major recent conflict, but otherwise it just doesn’t seem like something family would do for absolutely no reason.
I have no problem not being invited to a family member’s wedding, but this just seems odd. In our family, it would be the norm to preemptively tell certain people before the invitations went out why they should not be expecting one: the venue only accommodates 100 people, the budget is extremely limited, the bride’s family is organizing and they gave us a limited number of seats, your ex-husband and children specifically requested that you not be there, whatever, but say something.
The text response from the sister is so vague and bizarre. I would have responded immediately had it been one of my siblings. “The invitations went out in February”— so does that mean I’m not invited, did mine get lost in the mail, what’s up? Again, I would be fine either way, but I would expect a clear and direct answer.
Completely opposite wedding conflict... I was confronted on Christmas because I DID invite someone to my wedding. Yes, it was awkward and irritating that this person questioned my motives repeatedly, but it was better than having an argument right at/around the time of the wedding or having it discussed and debated without my input over the 6+months leading up to the wedding.