Do you live close by ?
Does she work, or have an activity to get her out of the house on a regular basis ?
Could you pop round? With book, or some nice tea, some bath stuff, whatever really, it's just to have an excuse to drop by.
(She may not let you in, this could be because her place is in a bit of a mess, which happens with depression and she could be embarrassed), if that's the case, don't insist just leave things in front of her door and send her a text.
If you don't live nearby, send her a letter, or a parcel, or a cutout of a newspaper article that might interest her.
If she's not answering calls or emails your first step could be to try and re establish some contact, let you know that you are there for her. Texts may be easier for her, she can read them, but doesn't have to reply. She may not outwardly acknowledge anything. This would be hard, and it's okay to get frustrated, but try not to show her your frustration.
Try to have regular contact for a while before suggesting she gets help, if you suggest it again too soon she may feel like you have conned her into reinvesting in a relationship with you just so you can badger her to get help.
Take things step by little step, depression is a longterm chronic illness, and the road to normality can seem endless.
You can't cure her depression, all you can do is try to make her feel loved, and make the pain more bearable.
To want to seek help, she needs to feel that there is at least one thing worth feeling better for. At at the depths of depression, finding the smallest thing to feel positive about can be like climbing a mountain.
It's hard, depression is terrible.
Unfortunately you have to remember that loving someone is not enough to cure depression.
I wish you all the best with your sister, remember to take care of yourself as well.