Is it permissible for me to stay overnight at the home of my fiancée, whom I have not yet married, knowing that she lives with her mother and her elder unmarried sister, and that I reside in another city more than 400 km away from her?

If the Sharia contract has been concluded, and only the wedding and the payment of the dowry remain, then there is no harm in staying overnight; because by virtue of the contract, the wife is considered lawful for her husband. However, she has the right to refuse until the dowry is received. If there is a condition or custom that prohibits staying overnight or being in seclusion before the wedding, then it must be fulfilled. As for when the contract has not been concluded, it is not permissible to be in seclusion with the fiancée or to stay overnight at her place, because she is still a non-mahram. Staying overnight with her family can lead to temptation, as the suitor is a non-mahram to his fiancée, and it is not permissible for him to look at her except for the purpose of engagement.