Need to fin a book but i don't remember the name of it. What I remember is below: Can you help?
Thanks for your question! That could be so many things, and even though you gave us many details it's still a bit hard to narrow down. The closest Nora Roberts thing I could find was Time and Again (Formerly published as two books, Time Was and Times Change), about 2 brothers, one of which stays in the past, but they are both from the 23rd century and stuck in our time, and the books are no longer in our system, if we had them.
It could be one of the later Outlander books by Diana Gabaldon, as well, where various family members jump back and forth in time.
Other romance authors who dabble in time travel a bit are Jude Deveraux and Linda Howard, but they don't seem to have written anything with a sister and brother like you describe.
You gave us such a wealth of detail, I hate to ask for more, but it might help us get further. Do you happen to know what city it was set in? Many of these time travel romances end up in Scotland somehow, so if this book doesn't do that, it's a good thing to know. Also, were there any special things or circumstances that caused the portal to open for them?
We'll keep looking as well - thanks!