I agree with your perspective on this. If you confront it, she may be more needy since you've already attempted this with her once before. When you decided together that you would initiate contact, she became even more clingy in response to you drawing a boundary. So drawing more boundaries and/or attempting another confrontation may result in something different than what you want.

I think in this instance, it's ok to just distance yourself silently and to withdraw without saying anything. Don't answer the phone every single time, and if you do, make it very brief and say that you have things to do and then hang up. Don't answer her on Whassapp.

Honestly, it's a toxic relationship if you've lost friends as a result of her extreme neediness. That's causing harm to YOU and to your life. Not acceptable. Your sister needs to get a clue and back the heck off. She needs to let you live your life and make yourself available to her when you can and if you can.

So, no guilt here. Take care of YOU and your life. You don't owe her your entire life and devotion every minute of every single day. She needs therapy and a therapist.