Well, I am glad that I asked.
First, I just want to make it clear that these are pieces of clothing K purchased for Puppy quite some time ago. He wore them, has long since grown out of them, and they've been sitting in boxes with his other baby clothes. I'm sometimes a little gift-dense, but I would never return an item back to the original giver because I didn't want it. Heavens.
This all started because we were going to mail a gift for the baby. And I remembered these shirts and how much K liked them, so I thought maybe we could send them down for the baby to wear. But because they were going to be in the same mailing as the present, I wondered if they would be seen as part of the present and worried about, well, about all the things you brought up in the comments.
The truth is, I'm not very sentimental about clothing. I was cleaning out the kids' closet once and threw Puppy's coming-home-from-the-hospital outfit in a thrift store pile without thinking about it (I fished it out later). I attach a lot of meaning to other tangible things, just not clothes. So I'm thinking, hey, they're shirts that she liked, so let's reuse them. But this is probably a time when clothes aren't just clothes.
We're actually going to be doing this in person (we're seeing both of Puppy's first families (separately) on Monday). Which, as several of you pointed out without realizing it, solves the problem because I can just ask her if she is interested in having the baby wear them. So here is the revised plan: I am going to bring the shirts with me, but not have them in any way connected to the present we have for sister-baby. That way we can see how things go on Monday and maybe have a conversation, and I can emphasize that these are things I had saved because she had picked them out for Puppy and I thought it might be sweet if sister-baby could wear them, too. But leave it up to her if she wants to take them home or have them stay with us. Or take them and return them later, whatever. Hopefully that erases any re-gifting taint.
To answer a couple of other questions, the fact that the baby is Puppy's (half biological) sister is out in the open. And she has been talking about them as siblings, at least with us, so I don't think I'm overstepping anything in that regard.
Thank you for your insights. This has an interesting time in our relationship with K and I have little idea what to expect on Monday. Some things have been said and written in the last month that make me hurt for my little boy, her first born. I almost don't care about anything else that happens on Monday, I'm just praying that none of them get said again in front of him. He is young, but he listens and takes things in and believes every single word. And I don't know how to help him understand that none of this is really about him.