Sentence:  ... it was her own knife; little sister Mary had left it to her upon her deathbed, and she ought to have had it to keep herself long ago. But mama kept it from her, and was always letting Betsey get hold of it; and the end of it would be that Betsey would <WSD>spoil</WSD> it, and get it for her own, though mama had "promised" her that Betsey should not have it in her own hands. 
Meaning of the word marked with <WSD> tags:  To carry off (goods) by force; to steal. (14th-19th c.)