I have heard one version where Lady Erskine suggests Child Owlet murder his uncle. I don't know how "traditional" that version is, but that is the only way he would (as she suggests) inherit his uncle's holdings. Under medieval law, a wife was not her husband's heir, unless she were also a close cousin; she'd only have had use of a third of his estate for so long as she stayed single. For her to have had use of the full estate, she'd have had to marry her husband's heir, which casts some light on her possible motives. None of that, however, is ever spelled out.