A further thought:

Soun might disown Akane if Akane got pregnant by SOMEONE OTHER THAN RANMA.
So you
could use a mod of that story (forget the name but I liked it) where a
drunk Ranma impregnates a drunk
Yuka, and have R & A attend a party where Hiroshi and Daisuke spike the
punch.  Gosunkugi gets
Akane pregnant, and everyone knows the baby's not Ranma's.  This MIGHT get
Akane disowned
by Soun; it would certainly be believable that many Japanese fathers WOULD
do so.

Soun/Genma/Nodoka then want Ranma to marry Nabiki or Kasumi, obviously, to
carry on the Dojo.
Ranma refuses and insists on marrying Akane anyway.  Such a refusal MIGHT
get Genma and
Nodoka to disown Ranma.

This sets up your situation almost as you had it, with the exception that
the baby's not Ranma's, and
you can fix that by having the first baby die somehow, and set the story
later, with a four-year-old
child who IS Ranma's and Akane's.  The first baby dying (or whatever; could
be adopted away)
then gives you additional tragedy for R & A's lives.

R & A carry on as you suggest.  A gets pregnant.  A wedding is planned,

but before being carried out, a

Dojo challenger appears.  R & A lose due to R's trying to shield A while

A insists on fighting.  Happosai

disowns R & A as members of the school due to disgrace of losing.  No

Tendo Dojo for R & A to carry on,

so nothing to keep Soun from expressing exasperation with Ranma for

<something>.  Let's see...

what if, just after the above, Ranma's curse becomes permanent (only

reason why Nodoka and Genma

would disown Ranma) AND Ranma is seen by Soun as responsible for Akane's

receiving a Jusenkyo

curse (or something else bad; pregnancy doesn't count).  Such a

concatenation of events is approximately

what would be required to achieve the outcome your story needs.