Is it obligatory to shorten a very long hijab, front and back, that the questioner wears as an alternative to the niqab, which her father rejects, and to the short hijab, which does not provide adequate covering, in order to avoid the forbidden resemblance to nuns?

The hijab is obligatory for Muslim women, and a father must help his daughter adhere to it, based on the Almighty’s saying: "O you who have believed, protect yourselves and your families from a Fire." Avoiding imitation of disbelievers is obligatory. What is prohibited is imitating them specifically with the intention of resemblance. However, if something is not specific to them or if there is no intention of imitation, then there is no harm in it.