It would be completely wrong to lie to your sister and pretend he's still alive.
As you speak to your sister regularly and she always asks about him, she surely knows he has been ill a long time, has cancer/is very ill ? So of course she knows there's a possibility he would not survive. It will not be a huge shock to hear he has died; and you can emphasise your shared relief that his suffering is over and he's at peace.
When you've been to the funeral she will probably want to hear about it. It's a natural part of a loss, and an opportunity to remember your brother's life together. Don't deprive her of that ( or, yourself).
If she's very unwell herself, she has undoubtedly been thinking about her own .death. This is natural for older people as health fails. She may be glad of an opportunity to talk about that, express her own wishes etc.
Death is a normal natural part of old age; and by that stage of life people have undoubtedly dealt with it many times. It is healthy and normal to be sad; but also, to complete the circle by remembering the past, happy times etc.
Give your sister the dignity of treating her as your equal life companion .