The meaning of "our lass" seems to be interchangeable depending on who says it. There have been times when DH, who is from Yorkshire, thought someone was talking about their wife and they were taling about their sister. I guess it can be used for any female family member.
I use the terms my, or our [name] or so-and-so's [name] to distinguish between people with the same name. For example, my sister and DH's best friend's fiance are both called Julie, so I call them my Julie and Neil's Julie.