Did some poking around on the internet and found this interesting quote from a 1991 publication:
Quote: The average Japanese today sleeps with his or her children until the children are ten or fifteen years old, - one recent Japanese study found daughters still sleeping with their fathers over 20 percent of the time even after age sixteen. Even when the home contains a dozen rooms or more, parents and grandparents feel "lonely" if they sleep apart from the children in the family, and therefore go to bed with some child every night (the mean age in one study of children sleeping alone is 12.7 years).... Many families still practice what is termed dakine co-sleeping - with the parent or grandparent sleeping while physically embracing the child, a practice said to be beneficial to the health of the adult (I edited this paragraph slightly. -LP)
Anyway, I guess this answers my question. If this is the way it's done in Japan, then what you're asking definitely feels completely abnormal to her. The thing above about how the parent feels "lonely" if they're not sleeping with the child is exactly what she's been saying to you. You could be swimming against a cultural tide that's just too powerful to overcome... The question is, is she willing to compromise at all.