My sister-in-law has serious issues with me, which causes a lot of friction between me and my wife. She is always making rude comments about me behind my back, and my wife is always forced to try and pacify her. I think my wife has begun to believe some of the things her sister says about me, even though she says she ignores it. I have been married for three years and have tried everything to improve my relationship with her, but nothing has made a difference. If this continues, I am very sure there will be serious consequences for our marriage. I don’t know what to do. Please help.
The onus of fixing this isn’t on you alone, given that there are two other adults involved. You should confront them with the facts, either individually or together, and your wife needs to know what kind of toll this is taking on you. Family can always be difficult to manage, but it is important to remember that you and your wife are solely in charge of how your relationship evolves, and what it can withstand. Focus on strengthening that bond with your wife, talk more about what bothers you, and give her your perspective of how you see her sister even if it isn’t a nice picture. Also, if nothing works beyond a point, and you must take drastic steps to save your marriage, you and your wife should speak about that too.

I often wonder if one of my closest friends is jealous of me, and just hides it well. She is always congratulatory and happy in front of me when I celebrate some success, but it always feels superficial because she is never around when I deal with the difficult bits. I know she cares about me, but I also believe she cares about me only if I am not doing better than her. Am I overthinking this?
You are allowed to feel the way you do, and also allowed to draw boundaries that help you.