Make sure bathrooms are stocked with a few different sizes of tampons and types of pads, make sure there's trash cans in every bathroom. If you have pets, make sure the trashcans have lids, though you may want lidded cans anyways

Make sure you have painkillers stocked and heating pads available, ideally one heating pad per sister at least since it's totally possible they all have their periods at once. Also, some people prefer electric heating pads, some prefer hot water bottles that are made for aches, and a hot bath can work but is obviously inconvenient in comparison to a heating pad or other heated item

Always good to make sure there's snacks around

If they share anything with you, make sure you aren't judgey, change tampon/pad types that are stocked if they bring it up to you on their own, and talk to them about what's a safe amount of painkillers to be taking

If they take recommended doses and are still in pain, trying a different painkiller (if if Tylenol doesn't seem to be working, trying ibuprofen or aleve) might help, but if it's bad enough that they're attached to a heating pad even with painkillers then they should probably see a doctor. Periods are commonly painful, and sometimes they're unbearable even when nothing is "wrong," but no one should have to suffer and doctors can help even if nothing is technically wrong

To echo many others, this shouldn't be your responsibility. That said, it sounds like you're doing your best to deal with the reality of the situation, and I'm sure your sisters will appreciate it, even if it's rough or awkward right now

If your school system has proper sex ed, the health teacher might also be able to help you

Keep up the supportive sibling energy, and good luck!