Do I have to accept applicant in the order application received

[Ping W.]

Do I have to accept applicant in the order application were received? I have two applications, one is in a transition period graduation next year with a big family and two cats another one is military family with no pets. I prefer the military family with only one concern that there may be the possibility that they end their lease suddenly with a  permanent change of station (PCS) The military family was transferred to our area this year and they told me that would stay here for 2 more years. Do Navy change their orders all the time or the chance is low that they would be transferred again with this PCS period? Both of them are very interested in our house and pushing for decision. If I decide to go with the military family, how should I turn down the first one?

Any input would be appreciated!