Unfortunately it is very typical of toxic people to gossip and spread lies about you. That does not make being the object of a smear campaign any more fun, but at least you can rest assured it has nothing to do with you. Rather it is a reflection of your strength of character and possibly your ability to see through their facade.
If you are noticing behavioral changes around you, and you cannot explain them, it can be confusing. Depending on the type of relationship you have with the other person, you can ask them what is going on. Creating an open and inviting environment for feedback and sharing may give them to opportunity to confide in you about the smear campaign. That will give you an opening to defend yourself, although you will not want to become defensive. What I mean is that it will be far more effective to ask: ‘does that really sound like me?’ than to launch into a counter-attack on your in-laws.
That question is effectively your only real weapon against a smear campaign: does that really sound like me? Concentrating more on how you behave and interact with people will make it less likely that people will believe the lies that are being spread about you.
Keep your nose clean, and don’t stoop to their level. I know that may sound far from satisfying, but it is important.
If the smear campaign is (or has the potential to) interfere with your work or family life, then you will want to open up to the people around you. Have a private meeting with your manager to tell him your relationship with your in-laws in strained for example. You mentioning this, could be the reason she lets you know when the lies DO reach her ears. As with any situation, be careful who you confide in though, make sure there is trust. Going to a manager who already dislikes you, may just push him to believe the smear campaign more if he sees your sharing of concerns as a way to make excuses.