I'm going to answer a bit from the other side of things.
I'm 25 and the youngest of 4 kids. Yet if my parents need help financially, I'm going to bet an unequal amount will come from me because
Sibling 1 - no ambition. She had great grades in school went to a 4 year college when times were still good in most areas but her industry tanked around here. She wasn'twilling to move away so she just didn't work and lived with mom and Dad. She now works retail but has lived with both my other sisters for a while and now is back living with mom and dad.
Sibling 2 - Well 2 weeks ago she was homeless. Technically now she has a small apartment but she never has any money as she is too busy buying booze and drugs (literally she had a party with plenty of alcohol and drugs less a month before she became homeless...) She doesn't want to be alone so she gets loser jobs that bring her into this spiral. Then she'll clean up a bit when the guy leaves and start it all over again.
Sibling 3 - She is the best off of the bunch but she is working 2 jobs to afford the luxuries she does have with her family (included 2 kids 1 of which is a senior in high school now).
Then there is me. I will be finishing my masters degree this year and already make more then anyone else in the family. I don't have kids yet but I will and have quite a few luxuries for my husband and I, including
DVC, vacations, video games and systems, heck my husband has spent over $300 in comics in the last two months.
If mom needs help I'll pay. However I will fully expect the others to help out in other ways. To an extent this is already how it works. The oldest lives with them and does help them out with day to day stuff, espeically now that mom is on disability. The third sister is generally able to get out of work and help with driving them to appointments and doesn't mind spending time with them when they have to go to the hospital and stuff.
Me I'm bad at the people stuff. When mom had her surgery I did go up and be with her and help her out a few times when she shouldn't be alone yet but honestly it made me really uncomfortable and I wasn't as good at it as my sisters are. So honestly if something were to happen and Dad couldn't work I would be sending the checks and just going to visit, they would be doing the work.
This type of arrangement is ok in my mind. But if they weren't willing to do the work I would honestly be bothered. I would pay because I wouldn't let my parents suffer for my sisters fault but it would bother me because they made their choices if anything my siblings should all be much more financially stable as they have had much longer to get there stuff in order (the youngest after me is 35 btw) why should I be penalized for that, even if all I would be giving up is the extra vacation that year?
So after my long rant. I really think you need to find some way to do something to help out. Maybe look at it this way. Imagine for a moment you didn't have your brothers you were an only child. What would you do then?