Can you take a mortgage on a property if the other properties of the same color have a house?
You must sell the house(s) before you can mortgage the property. From the rules, page 4 (emphasis mine):


  MORTGAGES… Unimproved properties can be mortgaged through
  the Bank at any time. Before an improved property can be mortgaged,
  all the buildings on all the properties of its color-group must be sold
  back to the Bank at half price.