Give it a rest................no one is saying that the stepmother should have all of the father's parenting time while he's gone. What we ARE saying is that to cut the child off completely from the father's family while he's gone would be extremely detrimental to the child, especially given the circumstances that the father is in.
Which is why if they cannot come to an agreement on an improvised visitation schedule while he's gone, then the only resort is court. And given that the father is being deployed, that won't happen quickly enough. So if the father is requesting that the child spend all his parenting time with the stepmother and his extended family, but the mother doesn't want the child to go at all, then a compromise would be to have the child spend at least part of the visitation time with the stepmother and extended family. It's called compromise and negotiation...........same thing that's always done in custody disputes.
The OP asked for advice on how to handle it..........this is it. Compromise. Don't cut off the child completely from her father's family. At her age, she will not understand it and by spending some time with his family, she just might get to see and talk to him if the opportunity arises. But also given the situation, she probably shouldn't spend the entire ordered time (1 week a month) with her father's family. Personally, if it were me, I would still allow some time with father's family, if for nothing else but to give the child some continuity. But it would need to be monitored carefully to see how she handles it, give that father will not be there.
Anybody else got any better 'ideas', other than just projecting opinion? In other words, what can be done instead of what can't be done.......?