Would this bother you?

[Paige]

I don't think it would bother me at all. I would assume my child would tell me and there's no need for sister to go out of her way to inform me, especially since it wouldn't be happening for months. Sure, maybe it would come up in conversation but if she were distracted and forgot to mention it, it's no big deal. 

My cousins stayed with us several summers to find summer jobs in college since their parents' hometowns weren't the best. I don't know if my parents brought it up with my aunt and uncle first, but I'm not sure why they would. it would be a normal thing in my world. 

 

[Katy]

It wouldn't bother me at all.  But I would also be concerned the secrecy was a sign of a hidden agenda- social or possibly just manipulating for babysitting.

[Garga]

If I’d expected my college-aged kid to live at home for the summer, I would be sad to find out he wasn’t going to be home, just because I’d miss him.  I would also be irritated that my sister hadn’t said anything, solely because she’s so wonky about her own kid.  Coming from a reasonable sister, I wouldn’t be irritated; but this one?  Yes—irritated.  She likes to pull snowflake treatment with her own, but can’t imagine that you might have wanted so spend some time with your own kid during the summer? I could see where she might not have called you up out of the blue to mention it, as she may have assumed your daughter would tell you.  But she was visiting you, sitting with you face to face, and didn’t say anything? And she’s all special-special about her own child?  Yes, that would annoy me.

 

[Chris in VA]

My sil invited my son, who was 18 but a Senior in hs, on a trip overseas without telling me. That irked me, because did not know that going to this particular place was the dream of my other son. It really stung him. Second son said he didn't think she would offer that to him. He was in the middle of drug treatment. Broke my heart, really. But she didn't know about the longing 8n his heart. If she had discussed it w me I could have told her.

On the other hand, she also told my other son, when he was a minor teen, that he could come live with her--while he was missing from home (run away) and was under treatment for drug abuse. All without telling us. I was stunned and furious for years about that one. 

So that colors my answer. 

Yes, I would be annoyed. Yeah, your dd is an adult. Yes, it feels your sister is hiding it deliberately.  If it is because she wants to be a safe place for your dd to explore other beliefs, honestly it would still bug the crap out of me. I would not be grateful. I would feel undermined as a parent and might even tell her off. I wouldn't recommend that.