Yes, they are, and one reason is because they know that this life -- with all its imperfections and troubles -- isn't all there is, but ahead of us is heaven. Why, then, should we spend our time complaining? The Bible says, "We boast in the hope of the glory of God" (Romans 5:2).
I suspect we've all known people like this, and admittedly they're not pleasant to be around. But sometimes we need to look beneath the surface (so to speak) and find out why they are this way. Sometimes it's because they've experienced much unhappiness in life, and no matter what happens, they tend to measure life by what it lacks instead of what it includes that's good. Or occasionally I've wondered if someone was complaining just to get attention.
You may never discover why your aunt is this way, but that shouldn't keep you from praying for her, asking God to bless her so abundantly that she won't be able to find anything to complain about! Pray, too, that God will give you an opportunity to confront her -- lovingly but directly -- about her negative attitude.
Most of all, urge her to find her joy in Christ and in the promises of His Word. Life this side of heaven will never be perfect, but when we know Christ, we know we belong to Him, and He fills us with His love. The Bible says, "Do everything without grumbling or arguing, so that you may become blameless and pure, 'children of God without fault'" (Philippians 2:14-15).