A man – who is a member of the same household – is being treated for serious gunshot injuries at the Royal Victoria Hospital in Belfast.
The book of condolences will open at 11:00 BST in the school hall at Maguiresbridge Primary School, where James and Sara previously attended.
It has been coordinated by The Village Rural Development Association.
The group said it “feels it is important that our close-knit community have an opportunity come together and support each other at this difficult time”.
The vigil will be held in the school grounds at 19:00 BST.
Ms Whyte was originally from Barefield, County Clare, in the Republic of Ireland.
At the time of her death she was employed by Stormont’s Department of Agriculture, Environment and Rural Affairs (Daera).
She had previously worked at Lakeland Vets in Derrygonnelly, County Fermanagh where she was described as “an enthusiastic, hard-working colleague” who was devoted to her children.
James and Sara were pupils at Enniskillen Royal Grammar School.
A support facility for pupils, parents and staff will be open from 09:30 BST at the school’s Cooper Crescent site on Friday.
The shooting happened in a rural area about 75 miles (121km) west of Belfast and about eight miles from the county’s largest town, Enniskillen.
Two of the victims were declared dead at the scene on Wednesday morning and a third died later in hospital.
Police said no arrests had been made.
A suspected triple murder and attempted suicide was one line of inquiry for detectives, they said on Wednesday.
