You're adults. None of your parents owe you anything. So, no, there's no "should" here.
It doesn't matter how much money the parents have, how much money you guys have, how much the parents choose to help other siblings, etc. They don't owe you any help, any money, a tow for your car, etc. Period, end of story.
If you got married while you were broke, that's nobody's fault but yours. You should've waited to get married until you were in a better financial position.
Can't afford groceries? Get better jobs, get second jobs, go to a food bank. Your car dies? Call Triple A or put the tow/repairs on a credit card. Don't piss and moan that Mommy and Daddy should've bailed you out. They CAN if they want to, but they certainly aren't obligated to do so.
All of these things suck, yes, but that's what being a married adult is all about ... life is shiiiiity sometimes, but nobody's responsible for dealing with it but YOU. And, not for nothing, but it seems like you've had an awful lot of circumstances where you need "help" from the parents ... perhaps his parents don't want to keep bailing you out, because (a) then they'll be stuck doing it forever, and (b) you won't learn how to manage on your own if they do.
It's none of your business why they choose to help his brother and sister. It's their time/money and they can spend it however they wish. Maybe the brother and sister are more competent than you guys so they feel like helping them out once in a while won't spoil them; maybe they feel you guys are MORE competent than his siblings so you can figure it out on your own; maybe they're tired of your bratty, entitled attitude and/or your bad life choices, and therefore they don't want to reward your bad attitude/bad choices by giving you handouts.
It's pointless to try and guess why they help the siblings but not you, because they DO. NOT. OWE. YOU. HELP. in the first place. So you can feel jealous or spiteful if you wish, by all means, but it'd be a waste of your time because you're not being "denied" anything you're owed. You being pissy over this isn't going to make his parents magically start opening their wallet for you. You aren't entitled to their money or attention just because you want it.
Stop whining, and stop thinking that his parents owe you something, because they DON'T. Lose the entitled attitude, Princess, and get the hell over it.