Why did Mattie get invited to tea in Fever 1793?

It seems
that Mattie and her mother were invited for tea at the Ogilvie mansion because Pernilla Ogilvie
was feeling starved for conversation. Pernilla tells them that she has invited them over to help
put her in a better mood. She says that she is having a terrible summer. With most of the local
aristocracy out if the city for fear of the fever, Pernilla seems outright bored with a lack of
company. It appears that she has invited Mattie and her mother over to help pass the time and
provide some conversation as entertainment. As she says, "the rest of society has
vanished."

Mattie's mother had once lived the lifestyle of the Ogilvie's
before she was married. It appears that she knows Pernilla Ogilvie from this time in her life,
although they have not been in touch for many years. There are clues during the conversation
over tea that Pernilla thinks that Mattie's mother married down. She really only invited them
for tea because all the Phiedelphian's of the upper class have left and she cannot think of
anyone else who could come over.

Pernilla is at home with just her daughters
(and servants) which is likely why she included Mattie in the invitation. Of course, Mattie's
mother hopes to use this invitation as an excuse to enquire about matchmaking between Mattie and
Pernilla's son Edward.