slice I think you are going to be quite the wife, so ignore that silly pile of shit.
That being said, I had to deal with similar issues with my wife’s family who are used to getting their way. Like dogs, you need to train them firmly and always let them know who the damn Alpha dog is or they will try to run you over (not the nice ones, but then again your not worried about them).
My slutty shit-eating SIL showed up the day of my wedding (which she was a part of) saw my best man, and seriously told my wife that “she was marrying the wrong guy and should go after him instead”. Her advice ignored, she (who is married with 2 then, now 3) then proceeds to chase my poor best man the entire weekend, openly, in front of her husband and her entire family. Said husband was not amused, and best man was wisely not interested in her skanky ass.
The night of the wedding, she was more and more obvious to all as she got drunker, even chasing him to his hotel room. He shot her down, again. Before you feel sorry for her husband, realize that she was seriously engaged (real close to the wedding) to another man, while he was chasing her, flying cross-country to crash her girl’s weekend, ect. He ended up knocking her stupid ass up, and the first engagement was cancelled. So he cannot act surprised.
So I cannot stand her, and she knows it. Her husband’s a dolt too. However, her kids are great, and we love them, and so I hide my disdain for their parents from them.
My MIL and FIL are used to constantly getting their way (usually by guilt or complaining) and it was a rough patch for a bit while the power relationships were worked out, but now we get along really well. Polite but firm, with occasional losses on the “I would rather not do this” front, but I didn’t need to win every battle, just the war.
FWIW, be polite but firm from the beginning. As far as the SIL, a heaping dose of frosty disdain should do the trick. Luck!