Maybe I start by saying that I am pretty sure Kamila (among juniors) and - in particular - Alina are very popular in Russia and whatever I say is not meant to deny this fact. My remarks are of general nature as I see this question of YT views recurrent in this thread.
THE most popular is always tricky because it really depends on how you define "popularity" and subsequently how you measure what you have defined. Using different definitions and different indicators you quite often end up obtaining different results. Number of YT views is quoted often because it is very easy to obtain - you just check the YT and it is shown there. Still, it is not very informative because it says nothing of the number of viewers. You can imagine a small group of die-hard fans watching a clip 24/7 and arriving at the same result as a huge number of people watching it just once each. For the same reason it can be very easily manipulated by PR agencies etc. so it is not very reliable. Therefore it is not considered a very strong indicator. Number of Instagram followers is a stronger indicator because it shows the number of "unique users" i.e. individual followers. That's the difference. Indicators based on the number of individual "unique" users (accounts, IP addresess etc.) would usually be considered stronger and more reliable as more informative and more difficult to manipulate, although, again, it all depends on your definitions and purpose for which you need the numbers.
In any seriuos study you would obviously use more than one indicator to arrive in somewhat reliable result.
NO offence meant to any skater or their fanbase.