I have one aunt who was invited. She told me to call her on it. So, i texted and asked when the wedding was going to be so I could add it to my summer calendar. She responded that all the invitations went out in February. That was it.
Your sibling (the groom's mother) replied that the invitations went out in February w/ no other explanation added to the text?
W/o knowing your family, I would probably assume one of the following:
1. This one sibling is upset w/ you & your other siblings for some reason over something.
2. Since the bride's family is paying, perhaps, the groom's family were only allowed a certain number of invitations & your sibling is not able to invite everyone that they'd like to invite & had to very selectively choose whom they were going to invite from your side. So maybe she's upset herself about the situation & doesn't know what to say or how to explain.
3. Even though the bride's family is wealthy, they are possibly limiting the guest list on both sides, &, for whatever reason, only the one sibling was invited. And, again, the groom's mother either didn't know what to say when you texted or felt that she didn't need to say anything & was maybe aggravated that you asked.
In answer to your original question, for me, whether or not I'm upset over not being invited usually depends. Most of the time, I'm happy to not have the obligation. However, I may feel upset and/or hurt depending on who else was invited to the event. In a event such as a family wedding, if I were not invited & other family members on the same "tier" as I were invited, I'd probably be hurt - like if other aunts & uncles were invited, but I wasn't or if the other cousins were invited, but I wasn't.
So, in your situation, I'd probably be upset too. And though no one really is "owed" an explanation, I feel the kind & gracious thing would be for the groom's mother to have maybe explained the reasoning.
In a family wedding where some family members were invited but not all, I would think there'd be some explanation. Are the children of the sibling who was invited maybe in the wedding? Is there a different relationship w/ this sibling than there is w/ you & other siblings?