I'm sure we've all met people like this–and like you, I've often wondered why they feel they have to be this way. I suspect most of them don't even realize what they're doing–although some do.
Such an attitude is wrong, however, and inevitably someone like this pays a price in lost friendships and a gloomy outlook on life. Why is it wrong to be so critical? For one thing, it usually blinds us to our own faults. In other words, we become so preoccupied with the shortcomings of others that we don't examine our own hearts and face our own sins. Jesus said, "Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye?" (Matthew 7:3).
It also is wrong because such an attitude is based on pride. A person who is constantly critical of others is really saying, "I'm better than you are"–and that is wrong. The Bible warns, "The Lord detests all the proud of heart" (Proverbs 16:5).
Pray for your sister, and pray too that God will give you an opportunity to confront her about her attitude, lovingly but firmly. Most of all, pray that God will open her heart and mind, and that she will realize her need of Christ. He can help her overcome her critical spirit and fill her with His love.