What should I do about my brothers demanding the sale of my sisters' apartment to cover the construction costs of my mother's house, which was built on top of my house, and their threatening me with my mother's wrath if I refuse, especially after I donated my own money to cover the construction costs in an attempt to spare my sisters this harm?

There is no objection to you being allocated a plot of land if your adult, mature siblings agree. There is no objection to you allowing your mother to build on top of your structure, and what she builds belongs to her. You are not permitted to retract your gift to her, and your mother owns her apartment without a share of the land unless you grant her that. Your mother is not allowed to gift her apartment to her daughters except with the consent of all heirs. You have the right to object and demand justice. If your mother insists on gifting or selling to her daughters, you do not have the right to prevent her, but you should advise her to be fair. If your mother is going to sell the apartment to her daughters, you have the right of pre-emption over it. In summary: You have the right to object to giving this apartment to your sisters, whether by way of gift or by way of pre-emption.