Your stepdaughter is hurting BIG TIME.
I'm so glad your stepkids have had counseling -- my dad died when I was nine, and even though we had a large, close family, the adults all assumed that we (the kids) would get over it. I never did.
I guarantee you that every day brings those kids a new way to grieve for their mother, even after the counseling.
Becca may feel, perhaps subconsciously, that messiness a way of keeping her mom in her heart, especially if mom = nurturing and messy while dad = strict and neat. She may feel that to be neat and orderly is to lose her mom all over again.
And another thing...Sometimes messiness is an inborn trait, not a choice...like being lefthanded. It can also be a comfort zone.
I am messy to the nth degree. Neat people -- those who believe cleanliness is next to godliness -- think I'm a horrible person. I'm not. I'm just messy. Some of it is visual organization -- I need to see my "stuff." It looks chaotic to a neatnik, but I know where everything is.
I'd suggest this:
Ask only that she get dirty clothes into the laundry and that she take no food into her room, as Marty said. Beyond that, close the door and say nothing more. Let her have her messiness in her private sanctuary.
One way to introduce neatness without invading her space is to have her help you organize another area such as the kitchen or bathroom. Take her with you to help pick out storage items. Give her ownership so that she experiences the reward and fun in organization. This is also a great time to encourage her to talk.