Is it obligatory for a girl to remove the overcoat she wears for added modesty, to please her parents who consider it an extremism, knowing that she wears a hijab and loose clothing?

If loose clothing fulfills the modesty required by Sharia, then there is no harm in sticking to it to please one's parents and avoid severing ties with them. The important thing is modesty, achieved by any type of clothing, as long as it meets the conditions of Islamic dress. However, if these clothes do not fulfill the modesty required by Sharia, then it is not obligatory to obey them in this regard, because obedience to Allah takes precedence over obedience to any creature. "There is no obedience in disobedience to Allah; obedience is only in what is good and right."