It's been like this forever. You can also ask Jaheira about Isobel's parentage and she will confirm it. Then if you say you need to talk to her yourself she'll tell you to go upstairs and get the blessing. When you already have done that and have the blessing.
The first time I encountered it was pretty funny. I had looted Marcus' corpse and apparently at some point the Harpers moved it outside. So I was telling Jaheira about him being upstairs while his cold naked corpse was lying outside the window.
