I take care of my father in law because he was diagnosed with lupus, will the military see him as my dependent?

If he requires care as identified by the medical services and he is dependant on you financially as well there is a good chance he can be considered a dependant. This issue can also impact where you may be assigned, and even warrant a separation for extended periods based on mandated assignments other than deployments. Thank you for your service.

Be sure to get them registered in DEERS. There is a form you can actually fill out to claim "secondary dependency" I believe it is still DD 137-3, it will go over all the documentation you would need for DEERS/Finance in the instructions. As long as he lives with you and you provide more than 50% of his support you should be fine. To consider here, does he get social security, disability, pension payments or long term payments through a legal settlement?