A non-Himachali cannot buy agricultural land. Since the economy of the state is agricultural/ horticultural driven, Himachal Pradesh has certain provisions under which non-Himachal residents and non-Himachal agriculturists cannot buy agricultural land in Himachal even if you want to start your farm or build your farmhouse.
However this is a settlement transfer from a mother to her own children, for this you may have to establish the fact sh is Himachali and your father had been staying or residing there for over 30 years.
The rule for purchase says that :
You first need to clearly determine the purpose why you intend to purchase or acquire under Rule 38 A (3) of Himachal Pradesh Tenancy & Land Reforms Rules 1975. The permissible limits for building a residential house is 500 square meters and an area not exceeding 4 acres for agricultural or horticultural purpose.
Whereas your case is transfer by settlement hence the rules for purchase canot be strictly followed in your case.
For more information you can also visit the following websites:
(i) Department of Town and Country Planning, Himachal Pradesh (http://himachal.gov.in/tcp/)
Chapter 10 of Section 118 of Himachal Pradesh Tenancy & Land Reforms Act (http://himachal.gov.in/tcp/chap10.htm)