Firstly, make sure your sister’s doubts do not infect you. You please be clear that Perumal is not causing any harm to anybody. He is simply permitting every individual’s karma to unfold. He is the ‘Director’ of our story, but we ourselves are the script-writer for our life. Whatever script/screenplay we have prepared for ourselves by our previous actions, He simply lets that unfold. Only when we surrender to Him completely, He may make some changes to the script; still he retains the overall flow of what we had prepared.
Secondly, for some time please do not try to convince her with your explanations. She may not be ready to understand. You are able to understand because of the people you associate with (either in person or through upanyasams etc). Likewise, her thought process will be in line with the kind of people she is associating (either in person or through social media and other virtual platforms).
நிலத்தியல்பால் நீர்திரிந் தற்றாகும் மாந்தர்க்கு
இனத்தியல்ப தாகும் அறிவு
Until she keeps associating with people with illogical intelligence, she will continue to drift that way. She will think you are simply sentimental; the more you try to explain to her the more she will explore other sources to counter your explanation.
Just don’t discuss spiritual topics with her at all. Simply pray to Perumal for her. Because of your surrender to Him, He will arrange situations for her to change her heart in this life.