Wow - sounds like your parents should be able to be doing better than they are, so most likely consumer spending is the problem. This is good, because it also means it is a problem that can be solved.
Dave Ramsey would be a good start - I don't personally agree with his politics, but he's a good motivator. You might want to give them a book or, if they aren't readers, see if he has a video DVD or some such.
You said they weren't very computer literate, so as far as budgeting goes, they need the envelope system. (Easy to do with cheap paper envelopes, but Dave Ramsey does sell fancier versions for, like, $10). This worked well for me all through my twenties - you pay your big fixed items (like mortgage etc) by check. Deposit a fixed amount into a savings account for bigger items that are variable or episodic. Take the rest (or the amount you have budgeted for living expenses after savings) and divide it into envelopes by category. I put groceries in weekly envelopes, gas could be weekly or monthly, entertainment was monthly, clothes monthly etc.
What it is great for, is you get to the end of the week, look in your food envelope and see you only have $10 left, so you go "Hmm, guess I'll make beans and rice". Keeps you from getting to the ned of the month with no groceries.
Same for entertainment. If you've only got $30 left in your envelope for the month, you might have to choose between dinner out OR a movie - not both.
With your mom's irregular income, it would be great if they could budget to pay their daily living expenses out of your father's income, and save mom's income for those irregular expenditures, retirement savings, or paying off their mortgage.
I know it's difficult to get adult parents to sit down with you and discuss finances, but you might have an "in" to this conversation by discussing the care of your disabled sibling. What happens to them if your parents die? Do your parents have a will, life insurance?
You'll need to address this in positive terms - in terms of how great it will be not to be stressed about money, to be able to retire, etc. Right now I suspect your mom is spending money to make her feel "better" about things, so you can't just yank that away and put her on a budget, without giving her something positive in return.
In Your Money or Your Life, they advocate making a big wall chart, updated monthly, showing the intersection between dwindling debt and growing savings.