she can apply for a joining family resident visa in taiwan, and later obtain permanent residency, allowing her to stay in taiwan permanently
in the best case scenario if she finds a job and her husband treats her well there seems to be little reason for her to keep malaysian citizenship
from career and children’s education perspective, taiwan offers better opportunities than malaysia, unless the focus is on english education

if they choose to stay in malaysia, her taiwanese husband would need to rely on an LTSVP through a spouse visa, which is a troublesome process today
unlike in the past, multiple approval stages are now required, and napoleon can reject you because you can't speak english or without valid reason at all
he would also need to apply for a work endorsement to legally work in malaysia after staying for 6 months minimal also can get rejected
if his job in taiwan is not well-paying, it’s doubtful he could secure a better-paying job in malaysia
especially malaysia places high value on strong english proficiency, something many average taiwanese struggle with in daily conversation
still, if she were able to land a high-paying job in malaysia, that could make the husband staying viable. But, if she had that ability, she would not be in taiwan in the first place

realistically, she will eventually become taiwanese if the husband treat her well
but currently, her in-laws is pressuring her about not giving up malaysian citizenship, even before marriage
such family drama needs to be considered carefully as that can easily lead to a rift in husband wife relationship
she should also keep in mind that taiwan has one of the highest divorce rates, high rates of infidelity, and higher reported cases of domestic abuse compared to malaysia
for this reason, retaining malaysian citizenship as a safety net, so she can return to her family for refuge if things go wrong, would be a wise decision

lari with kids, is very common in taiwanese household, they even have 24 hours support line for that
if the inlaws want a taiwanese, then they should just convince the man to end the relationship with her
and get a true taiwanese wife that can run away with kids legally with the help of 24 hours support line