Is it normal that my sister wants me to photograph her?

I'm an amateur photographer. I don't have an arranged portfolio or professional contacts or anything, but it's something I like to do and anyone who knows me knows it's a hobby of mine and may have seen some landscape or nature shots I've done. Anyway, recently my sister wanted to drag me out to a wooded area nearby and take some pictures of her in a loose dress. It was a pretty standard wannabee artsy girl thing so I did it. I don't know why she wanted them, but I guess she posts them online or something. I don't know, I don't follow her online at all. I love her and all, but I get enough of her in real life to not want to see her on my computer.
I did a few more of these generic shoots with her. Then she wanted to do it in her underwear. I thought it was a little strange but not really unusual or immodest so I agreed. Everything was tasteful so I had no problem with it. The second time she wanted to do a shoot in her underwear, which was yesterday, after a few pictures in, she took her bra off unannounced. I was confused by this but she clearly wanted me to keep shooting so I did. I found the whole thing to be strange. The only time I had ever shot nudes was a local model so desperate to beef up her portfolio that she basically allowed anyone in the area who had a camera to do an unpaid session with her. (She seemed like the type of model who was only in it because it's a paid version of the type of explicit pictures she would take of herself in a mirror anyway, but that's another story.) I don't know what my sister does with the photos, and if she was going to tell me she probably would have done that in the first place, and I don't know if it would be tactful to ask. Should I be more questioning of this?