Answer: They are if the property qualifies for an exemption
Explanation:
Generally speaking it is illegal to deny a person the chance to rent or buy a house based on their age amongst other things due to the Fair Housing Act of 1968.
There are however, some exceptions to this law and one such exception has to do with the elderly. On account of the special needs they have, elderly people homes and communities can be exempted from this law so as to allow only the elderly to rent or buy in a certain place.
This is contingent on certain requirements being met and if they have been met in the above scenario, the homeowners can indeed refuse access to the niece in question.