My [29 F] parents [50sM/F] are full on ghosting me?

So I've always been the least favourite child in my family. I was a below-to-average child and my sister was a goddam genius. Guess who was the pride of the family, woo-hoo!

But while I still felt that while there was blatant favouritism with my sister, I still enjoyed a semi-normal relationship with my parents. When I moved out, the transition was smooth and I never went back or asked for money.

We enjoyed semi-regular contact because we live in the same city and what have you. I thought we got along.

Then suddenly about 26ish days ago, they stopped answering my calls. I tried email, phones, mobiles, facebook, everything. They flat out ignore me, but I see my mother's constant activity and interactions with my sister and other extended members of my family.

I've tried asking uncles and aunts, and they just shrug and say 'ask them'. I called my sister, who asked my parents, and according to her they said that they're 'just busy'.

My sister found it weird that I didn't turn up to my aunt's large birthday party, and asked me why I wasn't there. It's because I wasn't invited and had no idea a party would be going on.

I went to their house, and while their car was in the driveway, their doors were locked and I couldn't see through the windows.

I'm honestly just thinking of giving up, but I keep wondering why they're doing this. There was no fight or reason that I could think of. I don't want to ask my sister to contact them for me again because I don't want her stressing out over this.

tl;dr: Parents (and possibly some members of my extended family) are ghosting me and excluding me from events. Is this normal? How do I deal with this?

[UPDATE: I called one of my aunts and she says she's having dinner with them tonight and will ask them directly. I have faith in her getting a proper answer because she's pushy.

My grandma said that they didn't even mention me, and another aunt says they haven't said anything about being mad at me at all. In fact, they were bragging about me a few weeks ago. So that's weird.]