Henry, a check of the Debrett's Online Database finds that you are correct. The heiress presumptive is now Lady Rachel Elizabeth SUTHERLAND (b. 1970), unmarried.
While the middle sister, Lady Rosemary Millicent (b. 1972) is unmarried, that is not true of the youngest sister ( the only full sibling of the late Lord Strathnaver). Lady Elizabeth Murray St. Clair (b. 1984) is married to Yacov COSTIN. They have 1 son, Isaac Sutherland (b. 2015), who is the next one in line after his 2 half-aunts and his mother.
A quick question-- now that Lady Rachel is the hp, why doesn't she carry the courtesy title of "Lady Strathnaver?" I refer to what seems like a similar situation with Viscount Mersey and his secondary title of "Lord Nairne." The hp to that title, his daughter, is called the "Mistress of Nairne."
Thanks.
Brooke