Although it may be tiresome for you, in reality you're already helping your sister-in-law by being a good listener, and I hope you won't stop. The Bible says, "We who are strong ought to bear with the failings of the weak and not to please ourselves" (Romans 15:1).
At the same time, ask God to give you the wisdom to help her become a stronger person, both emotionally and spiritually. I'm not a psychologist, of course, but she's probably a very insecure person who has little confidence in herself, and as a result she is easily hurt by what others say or do. She may even assume others are tearing her down when in reality they aren't (although it's probably hard to convince her of this). Such people can easily become focused only on themselves and their needs, and have a hard time thinking of others and their needs.
What can you do? First, let your sister-in-law know you care about her and accept her just as she is (which, after all, is how God looks on us). Then ask God to help you encourage her to think of others and how she can help them. The Bible says, "Therefore encourage one another and build each other up" (1 Thessalonians 5:11).
Most of all, encourage her to see herself the way God sees her -- as someone who is deeply loved and valuable in His sight. And this will happen as she opens her heart and life to Jesus Christ.