My mother always said
Ви as a plural to her parents, grandparents, aunts and uncles. My father always used
ти as a singular to his parents, grandparents, aunts and uncles.
I am not that young anymore and I use singular form for my parents and grandparents but plural for aunts and uncles. Generation of my parents, according to my observation, used
Ви in 70-80% of cases. My generation - 10-20%. I am originally from south west of Ukraine.
So in general I would say it depends on the region, religion and real amount of respect you want to put in your words. A couple of words about religion: what I have seen is - the more religious people the more common is to use plural form.
I would also agree with Chizh's comment - the tendency I see is to use more and more singular form.