Firstly, it is possible that you acquired the apartment as a gift, during your late mother's lifetime, or by way of an instruction in a will she made. As a rule, a person is free to do as he/she wishes with his/her own property, be it during his/her lifetime by way of a gift, or after he/she dies, by way of a will. This includes the freedom to leave property to anyone he/she wishes, including non-family members, or to divide it unequally between family members, or leave some out entirely, as he/she wishes.
If a person is of sound mind, and is not subject to duress, undue influence,or etc, and has the necessary legal capacity, then it is very hard for siblings to challenge the legality of any gift or bequest of an apartment made by that person. However, if a person is elderly,frail,ill,dependent on a family member or caregiver,or in a poor emotional state etc, it may be possible for the siblings to challenge the legality of the gift or bequest and get it cancelled. Much depends on the exact circumstances of the case and the weight of evidence that can be obtained by either side to support their respective cases.