That box clearly wasn’t his he stole it from his sister as you say - he took it and kept it as a trophie
“Killers like to take trophies and souvenirs from their victims. Keeping some memento — a lock of hair, jewelry, newspaper clips of the crime — helps prolong, even nourish, their fantasy of the crime. In my research, I’ve seen this happen again and again.
http://mindhuntersinc.com/why-killers-take-trophies/
Oh, absolutely!
He’s demonstrated how he likes to have things such as locks of hair, and keeping that box of Sheila’s which was probably given to HER but Grannie Bamber gave him a kick. He stole it because he was jealous, and it was also part of Sheila. He would have got excitement touching it knowing Sheila’s beautiful fingers had touched it, and then he’d replay the murders through his mind and get a massive surge of excitement reliving it in his mind.
Who knows what warped thoughts went through his mind about a Sheila long before he killed her? She was stunningly beautiful and although there’s not been any speculation that he attempted to sexually assault her, she did seem scared of him in later years and avoided him.
I also read (though aim not sure it’s correct as it’s not an official source), that Sheila had no knickers on when she was killed. As all women know, you always wear knickers when you’re menstruating, and Sheila had put to soak her worn ones she’d worn that day in a bucket, ready to wash the next day. When getting ready for bed she’d have put on clean knickers. So that’s weird...
And before some delusional jumps on that and comes out with the nonsense about ritual cleansing, no-one puts their watch back on to kill themselves
Now were any of Sheila’s clothes found anywhere in the house, including the washing machine, that were sodden with blood and gsr...