Woah, this thread got nutty!
Listen, I don't think my family is a train-wreck. And to be fair, for the most part, I think we do pretty well. The kids usually get along with each other. I usually get along with my step-kids. I love my step-kids and DH loves his step-kids.
Most of all, I do not consider my family an "experiment." They're my FAMILY. All of them.
The locking cabinet, the shrine to the ex-wife, the 11 year-old with the attitude problem, the girls crying -- these are not deal-breakers, they're life. And if I just wanted to throw in the towel, I wouldn't be here, seeking people's input. And as we all know, when you're in the middle of something difficult, it feels like it's taking over your life, even if - put into it's proper perspective - it's not.
So, back to the locking cabinet. To be clear, DSS already has a small locking BOX in which he keeps his allowance and a few keepsakes. I agree with previous posters that all children need a safe, private space for their treasures. I had a small locking box as a child and I loved it. We do not have a key to DSS's box.
What's he's asking for now is a CABINET. He saw one in a catalogue and pointed it out to DH. The dimensions are something like 4x3x2. It's pretty big.
In the grand scope of life, this is probably a battle I could choose not to fight (save for the part where DH hangs onto an extra key), but I don't like what it says for a member of a family to have such a large space in which to lock up belongings. I think it screams, "I don't trust any of you not to mess with my stuff, so I have to keep it locked up." I have a small, fire-proof box in which is locked important documents (birth certificates, SS cards, passports, etc), but other than that, I don't lock up my belongings. Until proven otherwise, I trust that my space will be respected.
Allowing one member of the family to have his own large space for locking stuff up feels like a step backwards in the "blending process." I think it reinforces not trusting one another, which is something DH and I are trying very hard to work through.