I would say...
Written by Anselm
(10/15/2009 6:51 a.m.)
in consequence of the missive, The whole family, penned by Barbara
...at least Marianne, her mother and Willoughby - the "we" would be most easily read that way. Besides, I could hardly imagine a project like that not including him. If Mrs Dashwood were involved, then I'd like to imagine that Elinor was as well, and perhaps even Margaret (remember Margret? She's the one who always gets left out by authors).