In sequence instead of random flashing?

It's most likely because the randomseed is picking the same number each time. Try this:

local Time = 5
local descendants = script.Parent.Lights:GetDescendants()

if Time == 5 then
    repeat
    math.randomseed(tick()) -- sets the randomseed to tick(), ensuring a different randomisation every repeat
        wait(math.random(1,3))
        for index, descendant in pairs(descendants) do
            if descendant:IsA("PointLight") then
                descendant.Brightness = .1
                wait(.3)
                descendant.Brightness = 1
            end
        end
        Time = Time + 1
    until Time == 5
end

Your script is changing all of the lights' PointLight Brightness at the same time, rather than randomizing which ones are set off, you can add another random sequence down into the table index like so:

local Time = 5
local descendants = script.Parent.Lights:GetDescendants()

if Time == 5 then
    repeat
    local timer = math.random(1,3)
        wait(timer)
        for index, descendant in pairs(descendants) do
        local randomsequence = math.random(1,2)
            if descendant:IsA("PointLight") and randomsequence == 1 then
                descendant.Brightness = .1
                wait(.3)
                descendant.Brightness = 1
            end
        end
        Time = Time + 1
    until Time == 5
end

This way, anytime a light is called in the function, it will either stay on or flicker. However, if you want the lights to flicker together you'd have to directly define them in the function instead of using a table.