The split you agreed to when Mom's needs first started rising was built on the facts you had then. Those facts move. One sibling picks up a new schedule, another's work gets busier, and the daughter carrying most of the labor now carries more of it. A yearly reset keeps the split matched to what is really happening, and it does this without anyone having to prove a thing.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Step one: pull the list of what was actually paid
Gather every shared cost and every shared task from the past year onto one page. Include the bills paid by one person and reimbursed later, the fuel for trips, and the supplies bought for Mom's home. Mark which lines were paid and which were only promised. The page is the record, and the record should be the starting point of the conversation.
Step two: recheck each share against the current work
For each share, ask whether it still matches the work behind it. A sibling who covered the monthly bills but now covers nothing has a share that no longer fits. A sibling who took on the weekly visits has a share that has grown without anyone writing it down. Put a number or a plain label next to each share, and compare it to the task list.
Step three: confirm who still has room
Ask each sibling, one at a time, whether they still have room for their share as written. Use a plain yes, no, or somewhere in between. You do not need pay stubs or a full account of anyone's life. A sibling who says 'less than before' has given you a usable line item. A no is also a line item, and it is far better to hear it at the reset than in a late-night group text.
Step four: write the new split and the rule for changes
Write the new split on one page with the date it takes effect. Add one rule: any change to a share is agreed in writing before the new amount starts, and any expense over the agreed limit gets a message to the group first. This rule protects the daughter doing the most labor, and it protects the siblings who give money. It is fair to both.
Step five: keep the reset from becoming a trial
A reset is a repair, not a verdict on the past. Keep the tone to facts and shares, and leave out who should have done more. If the past keeps coming up, write it as a separate line and leave it for a different conversation. The point of the yearly reset is that the split stays current, so the next year starts clean.