Hi Nancy!
The following is based on approximately ten years of personal listening and
observation:
-Some children will wean on their own during the subsequent pregnancy due
decreased milk volume (usually around the fifth month) and some will keep
nursing right on through the pregnancy.
-Some children will wean because they report to the mother that her
breastmilk 'tastes funny'.
-Some mothers find breastfeeding while pregnant to be: uncomfortable to down
right painful - and some say it was not a problem for them.
-Some mothers report prenatally or postnatally, an almost claustrophobic
feeling, or 'feeling touched out'; while others are fine until after the baby
is born and then suddenly they just don't like the way it feels to nurse the
older child.
-Somethimes the older child will increase his / her nursing big time
postnatally when the mother's milk 'comes in' and sometimes that is when the
older sibling will suddenly stop nursing because it was the suckling that was
enjoyed.
-Most mothers that I've known, that have tandem nursed, have spoken of a
special bond between the siblings - like they're sharing a very precious time
together at the breast or the mother will say something like "they look like
they're sharing a secret".
-Finally, the mother's attitude is everything!  She may very suddenly decide
during pregnancy that this is "not for me" and may be feeling very
overwhelmed, quilty, exhausted, or emotionally confused by raging hormones.
 She is apt to be in great need of support, encouragement and information to
continue tandem nursing, if that is her decision; or if not, to gently
encourage the older child to wean by decreasing breastfeeding frequency and /
or duration plus tips on how to distract the older child while remembering
the golden rule of weaning - "don't offer - don't refuse".

Hope this helps!
Leona Simoes, LLLL