Should the equalization among children concerning a gift be based on the original value of the gift (100,0), or on the current value of the apartments (210,0 or 270,0)? And should the difference in price between the two apartments (60,0) be included in this equalization, bearing in mind that the mother is still alive and possesses liquid assets?

The preponderant opinion is that it is obligatory to be equitable among children in gifts and endowments, unless there is a need that necessitates preferential treatment. If your mother's favoritism towards you two was due to a need, then it is permissible, and she is not obliged to revoke the gift. However, if the favoritism was without a valid reason, then it is impermissible.

Nevertheless, since the gift has already been disposed of by purchasing the apartment, the mother cannot revoke the gift. Therefore, her only option is to give her other son the equivalent of what she gave you (one hundred thousand).