I've just pasted in a whole bunch of links to info on the Posses, then hit the back arrow without thinking and lost it all. Shoot!!!!!
Do I feel like going through it again? Okay, here's the links, but I can't devote so much time on explanations.
Here's a link to a genealogy (in Swedish). Sigrid's (I can't be bothered to keep trying to spell her surname) paternal grandparents were Frederick Salomon Posse, landowner, and Magdalena Charlotta Bennet. The Bennet family, you will see, was originally Scottish but married into the Swedish nobility in the early 17th century.
http://web.telia.com/~u87534568/ant-627.htm
Here's a page in English on her PM uncle, Arvid:
http://www.answers.com/topic/arvid-posse
Here's a more extensive genealogy on the Posses:
http://freepages.genealogy.rootsweb.com/~corpusnobiliorum/posse.html
It has the following info on her parents (taken verbatim):
count Knut Lage Posse, orn 10 March 1821 at Hjälmshult, died 24 October 1900 at í„ngelholm, buried Malmö old cemetery, married 12 November 1852 at Malmö Lovisa Aminoff (born 21 December 1829 at Stockholm, died 5 February 1880 at í„ngelholm)
I confess I always have trouble navigating genealogies' complex numbering systems, but it seems Sigrid's siblings were Ebba, Arvid and Christer. It mentions none of them as being married to an American or living in New York. Maritha mentioned daughters - perhaps it was a daughter rather than a sister (as ET states) that Sigrid was on her way to visit? The page lists no children for her and Carl Johan, but then it is a Posse site.
Here's another genealogy that only mentions a brother, Arvid.
http://runeberg.org/adelskal/1923/0899.html
There is a great deal of info on all of these families on the web. The trouble is that a) it's mostly in Swedish and b) these families tended to reuse Christian names over and over again for centuries. Many Posse men had the names "Knut", "Lage", and "Arvid". Many Aminoff women were "Lovisa".
That's all the important stuff I posted earlier. It seems that, more recently, Sigrid's niece - Amalie Posse - was important as something or other. I THINK she was a writer and political figure.