Yes, the mother is a legal heir to the property of the predeceased son. The siblings of the said deceased son have a share in property so inherited by the mother and left intestate. This is the general provision in terms of Section 15 of the Hindu Succession Act:
"15. General rules of succession in the case of female Hindus.―(1) The property of a female Hindu dying intestate shall devolve according to the rules set out in section 16,―
(a) firstly, upon the sons and daughters (including the children of any pre-deceased son or daughter) and the husband;
(b) secondly, upon the heirs of the husband;
(c) thirdly, upon the mother and father; (d) fourthly, upon the heirs of the father; and (e) lastly, upon the heirs of the mother.
(2) Notwithstanding anything contained in sub-section (1),―
(a) any property inherited by a female Hindu from her father or mother shall devolve, in the absence of any son or daughter of the deceased (including the children of any pre-deceased son or daughter) not upon the other heirs referred in sub-section (1) in the order specified therein, but upon the heirs of the father; and
(b) any property inherited by a female Hindu from her husband or from her father-in-law shall devolve, in the absence of any son or daughter of the deceased (including the children of any pre-deceased son or daughter) not upon the other heirs referred to in sub-section (1) in the order specified therein, but upon the heirs of the husband."
The above provision is iniquitous to the widow and children of the deceased son. Hence the State of Kerala has since amended Section 15 as follows:
"Amendment of section 15.-In the Hindu Succession Act, 1956 (Central Act 30 of 1956), in section 15, after clause (b) of sub-section (2), the following clause shall be inserted. namely:- "(c) any property inherited by a female Hindu from her pre-deceased son shall devolve, not upon the other heirs referred to in sub-section (l) in the order specified therein, but upon the heirs of the pre-deceased son from whom she inherited the property.".