Q  My sisters and I inherited a house from my late parents. After some years, my sisters decided to gift their share to me. I moved to the house. Can they withdraw it in Sharia after 18 years?
It is invalid to undo the gifting if the gift was physically received by the recipient as seems to be the case in question.
Q  My niece is seeking divorce from her husband. At the time of wedding, she was gifted some jewelry by her in-laws. They are holding it. Are they allowed to seek this gift back?
If the relatives of the husband who hold the gift are her relatives as well, like cousins, then they are not allowed to ask for the return of the gifts. If they are not her relatives and the gift items were not used, then they are entitled to have them back.
Q  A father died, leaving behind his two daughters and a son as inheritors. One daughter told her brother that she will not want her share. After several years, the daughter demands her share from her brother. Does she have the right to it now?
If she gave her share to her brother and he received it, then she cannot claim the given share back for herself. If she did not give it to him but only promised him that she will give him her share, then she is allowed to change her mind; the same ruling applies if she gave her share to him but he did not receive it.