hi, i'm sorry to hear about all the problems you're having, i can completely relate (well, except for the illness, i may have medical problems, but you can't really compair that feeling unless you have the same thing) as for the in-laws...i definitely have the same issues. my mother is very nasty, to everyone, it's just her personality, some see it as funny, while others (those of us unfortunate to be on the receiving side of her attitude) think she's just pure evil. while she does have her nice moments (far and few inbetween) she does like to start with us (and we live with them). i swear she's not happy unless she's bitching! when she starts a fight, whether it has anything to do with me directly or not, she goes after me screaming and yelling (i guess venting) and when she gets nasty with me, i just say WHATEVER! and walk away, or just completely ignore her and go about my business. i've had at least 1 week without saying one single word to her, or her to me AND IT WAS GREAT! she was also much nicer afterwards. and when she gives me a hard time about my preference for my daughter's parties, or how i raise my child, i ALWAYS end it with, "well, that's nice for you, but this is MY child, and i have the right to decide what she can and can't do, eat, or goes. and if you have a problem with that, well, i'm sorry, but that just sounds like a personal problem that you just need to get over!" and i think that's the nicest possible way i can put that to her! sometimes she just says whatever, and sometimes she throws a tantrum (YES, just like a little child..without actually throwing herself on the floor kicking and swinging lol)
all i can say is try to keep your distance when she's giving you a hard time, do what you can to keep the peace, but never surrender to her wishes if it's something you're dead set against! and if your husband agrees with you, just blow it off as it being her problem, NOT yours...you need to think about you and your family over everyone else! IF you want to try and accomodate her and what she wants without adding more burden to yourself, i'd definitely tell her that if it is that big of a deal to her and the cousins to have the boyfriends, then THEY would have to pay, whether it's your mother-in-law, the cousins, or the boyfriends, but it WILL NOT be coming out of your pocket. and leave it at that....if you get the money, then allow them to come, and if not, oh well, not your problem.
hope all goes well, good luck!