All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If the deceased did not leave any heirs other than those mentioned in the question, then his wife gets one-eighth of the estate as a fixed share because the deceased had children; Allaah, The Exalted, says (what means): {...But if you leave a child, then for them is an eighth of what you leave, after any bequest you [may have] made or debt...} [Quran 4:12]
The remainder of the estate goes to the son and daughter by virtue of ta‘seeb (having a paternal relation with the deceased and not having an allotted share, so they get what is left after the allotted shares have been distributed); the male gets double the share of the female. Allaah, The Exalted, says (what means): {Allaah instructs you concerning your children: for the male, what is equal to the share of two females...} [Quran 4:11]
Hence, the estate should be divided into twenty-four shares; the wife gets one-eighth of them (three shares), the son gets fourteen shares, and the daughter gets seven shares.
If the flat is included in the estate and one of the heirs requested selling it, then the other heirs are obliged to sell it. The wife and the daughter are not entitled to demand dividing the estate or the price of the apartment equally among all the heirs because this means decreasing the lawful share of the son and implies going against the Islamic inheritance laws set by Allaah, The Exalted. The fact that they are financially unable to buy a house with their shares does not justify forcing the son to divide the price of the apartment equally among the three of them. However, if the mother is poor, then her son is obliged to spend on her and provide for her needs in respect of food, housing, and clothes. Ibn Al-Munthir wrote, "Muslim scholars held a scholarly consensus that the maintenance of poor parents, who have no income or wealth to spend, is obligatory on their child."
The same ruling applies to the sister if she is poor, and the brother is among her heirs entitled to inherit from her when she dies; in this case, he is obliged to provide for his poor sister according to the preponderant opinion adopted in Islamweb.
If providing for one's poor sister and mother is obligatory on him according to the Sharee'ah and he refused to spend on them, then he bears a sin for that, and such an act constitutes severing the ties of kinship and undutifulness to the parents. Please refer to Fataawa 87019 and 87856. In this case, they are entitled to refer their case to the concerned Sharee'ah court, if any exists, to oblige him to provide for them if he is financially able to do so.
Allah Knows best.