To. Monica :
1. You can stay in that flat, as long as you wish and your siblings have no locus standi to get you vacated and sell off the flat. You and your siblings are the legal heir to your fathers property.
2. IF your siblings have to stake claim on their share of the fathers immovable property, then they have to file for "Letter of Administration" (LA) from the competent court, after which they may stake claim on their part of the property, BUT UNDER NO CIRCUMSTANCES BEFORE THAT.
3. There is no such thing as "default claim" on immovable properties of a deceased. Neither they can sell their share of the property to anyone, UNLESS the above court decree is acquired and their is a settlement deed that may be filed during the proceedings of the LA.
4. IF the flat is transferred in your favour by the society, by virtue of the Nomination Form executed by your father, THEN ultimately, as Nominee/Trustee, you should be ready to give part possession of the flat or appropriate money ratio of the flat's market value to your siblings, BUT all this only after the siblings have initiated the Court proceedings for all this.
5. As far as possession of the flat by virtue of yours fathers nomination, there is no Limitation Act applicable. You can hold the property endlessly and your siblings are entitled to PART PORTION of the property as mentioned in point no. 2, above.
6. The society is under no legal obligation to inform anybody and / or contest its members nomination form. The society has done its duty rightfully, under the MCS Act, and the society cannot be faulted on this and neither your siblings are entitled to direct or restrain the society for anything whatsoever, BUT they can do so via the above court procedures only.
Keep Smiling .... Hemant Agarwal