Well... I don't think I'm old yet; but I agree with velochic. She's spot on; as usual.
I am totally aghast at this thread. OP, you said that your MIL is being rude and critical of you. She doesn't not treat you with respect. Exactly how have you treated her?
taking someones help as a gift or a loan and not even acknowledging it... that's disrepectful
taking thousands of dollars of money from someone; and then resenting them because you paid their portion of their takeout bill when they came to visit you! That's something else.
I'm not saying that she has a right to treat you like crap. But you don't have the right to be mean to her either.
Your reality is different than others', though. I can't even begin to state my opinion without being offensive, so I won't even try. Seriously. Your lack of appreciation for what they continue to do for you financially is almost vulgar. And yes, I think it's an entitlement issue for kids like yourself. I'm on your MIL's side here (and don't really care if anyone accuses me of that made-up word, "ageism") -- if I had a DIL that resented me for all of my help (I'm old enough to have a DIL), I'd probably make some passive-aggressive comments, too. Buy her some damned orange juice and let her eat your yogurt. Maybe if you did these things gladly, your MIL wouldn't have to feel so defensive. They have a right to spend their golden years doing whatever they want... not supporting you. This is an issue where there is no way to know what is the chicken and what is the egg. Have you considered your MIL's feelings at all? Maybe if you do that first, her passive-aggressive comments go away. Perhaps she picked up your "what we have is ours and what you have is ours" attitude from the beginning.
ETA: What I'm saying that even "your" side of the story makes you out to be ungrateful. I can't imagine how your in-laws are feeling. They tried to help with a house, they got the short end of the stick, now you resent them eating your yogurt. There is a disconnect that sounds very, very selfish... on your part. I absolutely can see how your MIL might feel hurt by your actions. You've let them take financial blow after financial blow on your part and you still resent that they are retired (and will need to live off of that retirement for another 30 years hopefully) and saving money when you can't. It's really such an immature attitude.