It really varies. One thing you can do is contact the school, or suggest the kids talk to a trusted teacher or the guidance office. The school may be able to intervene on things like no lunch and academic struggles. They may need to know the home situation to better support the kids. They may (as mandatory reporters) talk to CPS.
Your parents need to understand that you are their aunt and short of taking custody (which can be its own issue) you are not legally responsible and have limits to what you are able to do. Why are your parents not intervening if they feel so strongly? Are they unable to offer to take custody, talk to the kids, talk to your sister, etc?
Just because your family is doing well doesn't mean you can also take on other burdens. Now, I know that it's hard with kids but I'll give you this example. I have a relative who has a young child. This relative hasn't been a great parent, but is an OK parent. What I do (and what I can do, being not this child's parent) is I try to offer the child a soft place to land. Come play here. Have a meal here. Call me if you need a ride. But that doesn't mean I feel like I need to take the child into my home at this time, or that I even should. It's not cut and dry. If they are in danger, let them call you or the cops and ask to come to your home. If you think they are in danger, call CPS.
It is hard when you feel like you are taking care of someone else's responsibility. I would challenge your parents how they would then help YOU if you took on 3 kids. Would they take kids to/from tutoring? Help you with the grocery bill? Give you respite care help? Would they back you up in court if you had to go there to get legal guardianship? As a SM whose husband had physical custody, I get that it's not easy to pick up someone else's slack, and it can make you really resentful and tear your family. It's not an easy choice. There are a lot of factors here, including what it would do to your household/marriage and how you would be able to weather that.
I'm not saying give up on them, but I think you need to know more about options B, C, and D, too. And your parents need to understand that there are limits and boundaries and if they feel so strongly, they need a horse in this race, too.
ETA: When we have the sks' friends come over suddenly, pasta is cheap and easy and filling. Buy spaghetti and sauce when they're $1 and have them on hand. Eggs are also cheap and easy. Encourage the kids to get jobs for their own lunch/pocket money when they are old enough. Teach them simple meals so that when there is food, they can cook or pack their own lunch. Help them get bank accounts, perhaps in joint with you, so that their mom can't wipe them out. You assume the school knows, but they may not have the full picture and honestly, I think that going to the counselor and saying you are concerned may be a good way to inform the school. They know you pick them up, but do they know the extend of everything else? If there are any after school tutoring opportunities, encourage the boys to attend. My stepson's school had tutoring for kids on the sports teams, to ensure they all got at least a 2.0. Talk to the boys about asking for help from teachers, being more responsible than their mother (good bedtimes, for example) and encourage them to dream and have goals and a reason for doing homework, studying hard, etc.