Hi Delia,
That's the info I got from Des Hickey and Gus Smith's book, "Seven Days to Disaster." They got their information from correspondence with survivors, including fellow Pearl nurse Alice Lines. Alice didn't mention this in her earlier accounts, so it's possible that Greta may have invented it or it's something Alice heard and misinterpreted.
I haven't been able to find other information to confirm that Greta had a brother on the Titanic, so I might have to remove it as being an inaccuracy.