A family of two brothers marries a family of two sisters.

Two women who planned to be governesses their whole lives marry two Weston males.
Just think of how many orphans or single-parent children their are--and how many parallels there are among them.
You mentioned Frank rescuing Harriet; he also rescues Jane in the boating incident. Mr. Knightley rescues Harriet at the ball and he rescues her from deeper heartache by asking Robert to take papers to John while Harriet is there.
I've only mentioned a few...