I wouldn't walk away from a house simply because it has dropped in perceived valued from $180K to $150K. There are a TON of houses that have dropped that much, or more - you are not alone. We paid $211 for our house, dropped $30K in adding a 4th bedroom and full bath while finishing the basement, and the house recently appraised for $190K.
Two thoughts regarding your situation - you mentioned that your property value has gone down, but your taxes have gone up. How is this so? What does the county appraise your house at? I've challenged county appraisals before and had a property reduced from $9x,000 to $6x,000 and all it cost me was a $400 bank appraisal. Well worth it. If you could get the county to lower the taxes on the house it would allow you to pay it off faster (by paying more on principle) and could help get you closer to the break even point.
Second, how much do you owe on the house still? What is your interest rate? I would explore all financing offers to get the absolute best deal to again pay off extra principle and try to get to the break even point.
I, personally, would never allow any family member to buy a house for me and in return pay them rent. Too risky and stressful.