I hereby submit my SIL as a very frustrating case of dumbass-ness.
She has zero financial skills. DH and I are quite sure she doesn't understand how banks work. Literally, like she doesn't understand the difference between checking and savings accounts and definitely doesn't understand the concept of auto bill pay.
She went to university to get a degree in performance piano. She racked up more than $80k+ in student loans, which her mom, my MIL, cosigned. FIL (divorced from MIL) saw the trainwreck coming a mile away and refused to cosign any loans with her, but MIL couldn't let SIL not go to college so she signed her financial life away. SIL graduated 5 years ago but has never consistently paid her loans (doesn't even pay the minimum when she deigns to pay at all), even though she has a pretty sweet gig playing pianos on cruise ships. This is hugely impacting my MIL's credit, who still has 3 younger children to put through college in the next few years.
SIL spends her time cruising exotic locales, buying luxurious clothing (for her job, of course! Can't be seen performing in the same glittery/slinky/outrageous evening gown twice in one month!), doing all the dumb tourist money-trap activities each week (ziplining, exotic expensive restaurants, etc). For someone whose job pays for room and board for the duration of the contract (!), and feeds you breakfast, lunch, and dinner, she saves NOTHING. Her living expenses SHOULD be low (and because of these perks, the pay is correspondingly low too), but she constantly racks up credit card debt (using a card that has foreign transaction fees!! WHILE ON INTERNATIONAL CRUISES. WHAT.) to party with her crew friends.
A few years ago, she started refusing to pay for her portion of the phone bill (MIL had her on her family plan). She was blowing through (roaming!) data like nobody's business and her portion was at least $100+ PER MONTH. My poor MIL called me, sobbing, because she had to dip into college savings for the younger sisters to pay this outrageous phone bill that SIL had stopped contributing to for the last 8 months--and it came out that previous to that, SIL would use that same $100 worth of data, but only send something like $30 per month and call it good enough. I told MIL to kick SIL off her plan, but MIL was horrified at the idea; SIL is often out of the country and leaving her without a phone was unfathomable. Also, part of it is that SIL is very touchy and would absolutely stop speaking with MIL for a year if MIL gave her even tiny consequences for her actions. I dropped it because MIL was getting so agitated, and I honestly have no idea how that all got worked out, if at all.
SIL treats MIL like an ATM with unlimited money (and gets outraged when the ATM shows signs of reluctance) because she deserves it for going through the trauma of her parents divorcing when she was 7. The kicker is that MIL and SFIL don't make all that much -- maybe they bring in $40k per year, if that, and have almost no retirement savings, but SIL thinks they are rolling in money and are selfish if they don't subsidize her lifestyle.
MIL and DH often try to talk with her to get her to take care of her bills and basic responsibilities. DH wants her to refinance her loans out of MIL's name but she can't get her shit together and do it because it's "too hard". She likes to use her time at sea as an excuse to ostrich and stick her head in the sand and not deal with things because "she can't get in contact". Bullshit, she has enough contact with the outside world to post a bajillion selfies in exotic places wearing a new swimsuit/outfit each time. She just refuses to deal with the scariness of finances because they are overwhelming and she shuts down completely.
Each year she also doesn't get her shit together for filing taxes and files for at least 2 or 3 extensions. Mostly because she can't be bothered gathering the few various documents she needs and "doesn't feel like it" <head explodes>. Again, taxes are big and scary so she ostriches. She messaged DH a few weeks ago and asked/demanded he do her taxes for her because her extension is up in like 2 weeks. DH told her "No way. I'm not touching that with a 10 foot pole." lol
SIL is 3 years older than we are, but we are MILES ahead of her both financially and maturity-wise. I just cannot fathom taking advantage of family generosity/support like that and actively trying to be as much of a financial and emotional burden as possible. Within 2 months of having full-time jobs after college, both DH and I refinanced our loans to be in our names only so our parent's credit would no longer be affected. We have our own phone plan and pay it completely ourselves. We pay our taxes judiciously and file well before the deadline, every year.
I would have no problem simply letting her experience the natural consequences of being a complete dumbass and jerk, but the problem is my sweet MIL. She has tied herself financially to SIL and cannot extricate herself, and is sinking slowly because of SIL's ineptitude and selfishness. MIL made quite a few poor choices to enable SIL, but now seems to want out, but there's no way to get free without SIL's cooperation. We try to help out and cover some of MIL's and SFIL's bills, and help DH's younger sisters with books and a bit of tuition, but I get so steamed that each gift is in some way helping to subsidize SIL.