Is it possible to try broaching it in another way? My uncle actually had a similar attachment to a bed that belonged to his late wife. When he remarried his new wife put her foot down and her ended up having it turned into a bench that sits at the foot of his bed (basically the headboard is the back of the bench and the foot board forms the legs, he found a great furniture maker and it looks really nice). How do you think she'd respond if you said "I know you want to honor your brother, I'm wondering if we can look at other options or ways to have the bed in our house without it taking up the same amount of room."

Would she be open to using part of it now and storing the other part until you have a bigger place (depending on what the bed looks like maybe it could function differently in a smaller place as seating, wall art, etc.). Is part of the issue storing it at her parents (in other words would she be more comfortable with a storage unit you rented or in an apartment building you lived in). It may be something tied to leaving it with her parents and the pain she knows the whole thing could cause them, or not trusting them to not get rid of it.

If she reacts poorly to that or is horrified at the idea of altering any of it or dismantling part of it then you know she isn't ready to move away from the bed without more serious help (I mean therapeutic help) or she isn't ready to move in with you. In which case it is time for a gentle conversation about how her attachment to the bed is preventing her from moving forward in her life.

Out of curiosity, does she ever sleep over in your bed, sleep in hotels, other places when traveling, etc. or does she believe she actually needs the bed to sleep at night?