I have 2 SILs:
PoisonousSIL I have written about a lot. It has been more than 10 years since we have met in person but she still cannot bear to accord me any politeness. From the very beginning she decided I was somehow dangerous to her standing in life and treated me accordingly. I think it was only made worse by the fact that I understood why she disliked me, told her that it didn't matter and that we could try and be civil for family gatherings. But no, she just went to work on her DH, my DHs DB... made it impossible for them to be friends too.
StonerSIL is more insidious, DH still isn't sure that his little sister is a bitch, just that in choosing to be a persistent 'smoker' she has lost the plot a bit. That she is gay is of no consequence, but her persistent pouring of vitriol is. I lost interest in being polite when it became obvious that both she and BIL were angry that their DB was not the useless waste of space, lackwit, fuck up they ahd always considered him to be.
When he said he wanted to do an Access course and then an engineering degree they all (MIL and SFIL included) tried to dissaude him. He was, apparently, giving up a perfectly good job (NMW in a factory); too stupid to do a degree (yeah, that one flew well) and, the best, being brainwashed by a snobbish bitch who just wanted him for his money - not sure about the logic of that one, but it certaily explained how they saw me!
I know I am not like them. I am not an outgoing, uber party animal with hollow legs. I was ill for years (ME/CFS) and I have always suspected they saw my non attendance at weekends as a personal slight. DH used to go, explain, again, that I was ill, get the eye rolls, sympathy and a pint 
Now they just think we are too big for our boots. We have a house, 2 cars and a pleasant enough lifestyle because, even though I have not always managed to be in full time work, we have not Bought for Britain nor holidayed somewhere exotic every year for th last 30 years.
Sory, that was an essay. But they are at the forefront of my mind as I have started a business and I am waiting for the snide contacts to start.