There's quite a bit to it, but the short answer is: Possibly.
First thing you need to do is call your insurance. You will not get a 100% answer here.
Then, they must qualify to be dependents of yours. This means they must not make any income besides social security. They can't file a joint return. They must receive more than 50% support from you. There are a few other check marks, but if they fit the bill, then it's still up to your insurance.
Talk with HR to see if they can get you some reading material or a number to find out the specifics of your incoming insurance plan...This is where the final say will come from, and they may have even more stipulations.