There is no good reason you can’t learn violin online. There are some very lovely tutorials and how-to’s out there to guide you. If anything, I’d say the amount of free and low-cost resources available would make it way easier than shamisen which is an investment even on the “lower” cost end.
A lot of times practicing in front of a mirror can help with “bad habits” like posture. If you get a decent beginners book, you’ll be alright. I’m not saying an in-person instructor wouldn’t be better, but it’s not wholly necessary for learning. I know this wasn’t the advice you were looking for exactly, but if you’ve got a will there’s a way to learn, regardless of people’s apprehensions or prejudices about learning “properly.”
My source is playing violin and having formal instruction and watching online instruction done for the same instrument when schools were closed during pandemic. Don’t let in-person instruction gatekeep your passions. Violin is a very rewarding instrument if you put the time and effort in.
Edited to add: I have only just started shamisen so I don’t have a lot of helpful advice specifically for shamisen yet. Hopefully someday. Good luck! 
