Both my brother and sister in law are compulsive liars, which has led to us being virtually non-contact with them.
In my brother's case, it was always to make himself look more important - he'd blatantly lie to family/friends etc in front of me, saying things we'd done or said, when he knew I was there and I knew he was lying (and he knew I knew!), but he just did it time and time again. All that led to going low contact when I was around 18 and started doing my own thing instead of doing things with him as family.
With SIL, she has a chaotic life, and just lies to divert blame from her own mistakes - everything is always someone else's fault, even though it's blatantly obvious that it hers! Like the time she sold her house - she'd been telling us for a couple of years that her boiler was faulty, and then it packed up completely, but she lied to the buyers/solicitors when she sold the house, signing that the boiler was working, and then, when the buyers claimed against her for the faulty boiler, she continued to claim it was working, and even whinged to us that they were trying it on, apparently completely forgetting she'd previously been complaining to us that the boiler was knackered! She did the same with another house that had dry rot - she hid it, claimed no problems and then again the buyers claimed against her for rectification - she again, in front of us, claimed she didn't know, but for a couple of years previously, she was telling us her hubbie kept having to go under the floorboards to spray dry rot treatment on the joists! Same again with selling a knackered car - we all knew it was knackered, but she didn't disclose it and whinged when the buyer wanted their money back! Every single time, she claimed, to us, that she was the victim, and clearly never remembered she'd previously told us about the faults etc! When we pointed it out to her, she just denied it and accused us of making it up!
Liars seem to have very short memories! At least if your going to lie, remember what lies you've told, otherwise you look stupid!