If a relative, then I would just consider it a cost sharing arrangement and leave well enough alone. You're not making a profit and never will, since what you're being paid most likely doesn't even cover the mortgage interest.
Otherwise, for taxes you "HAVE" to designate a percentage of the floor space as exclusive use. Weather it actually is or not, is irrelevant. But I still recommend "don't go there" since it's a relative. You're just creating more unnecessary paperwork that could very well start screaming "HEY! IRS! AUDIT ME NOW! PLEASE! HURRY! QUICK! FAST!"
