Be cautious about jumping to conclusions about your sister's motives; after all, elsewhere in your letter you mention that you and your sister live in different parts of the country and seldom see each other.
Even if you have reason to question her claim, however, it may not be possible for you to persuade her to reconnect with her old church. After all, you can't control her life, and if you try (as brothers and sisters sometimes do!) it will probably accomplish little, and may even upset her. Perhaps the best you can do is let her know you care about her, and that you hope she won't cut herself off from her church completely.
Remember: Our greatest spiritual need is to grow in our faith - and one way this happens is through our fellowship with other believers. But it also happens as we hear the Bible taught and preached. Is this happening in your life? The Bible says, "Your promises have been thoroughly tested, and your servant loves them" (Psalm 119:140). Can you say that about God's Word?
Be grateful your sister watches a service on television each week; these can be of great benefit to those who are elderly or otherwise unable to attend services. And pray for her, that whatever her situation is, Christ might become her daily companion - and yours as well.