You may need a lawyer.
If this was a gift, and if the gift documents do not say otherwise, then each sibling owns 20% of the house and is responsible for 20% of the taxable gain (profit) on the sale. The fact that some are in higher tax brackets than the others is tough beans, in my opinion. The only way for them to pay less tax is for them to own a smaller share of the house.
When you sell the home, you will owe capital gains tax on the gain -- the difference between the sales price and the adjusted basis. Because this was a gift, the cost basis is the price your parents originally paid for the home plus the cost of any permanent improvements that have been made over the years.
For example,
Parents bought the home in 1980 for $100,000 and spent $20,000 on remodeling over the years.
Gifted to 5 siblings in 1992.
Sold for $225,000 in 2017, with a 6% real estate commission ($13,500) and $2000 in county transfer taxes.
The total gain is (225,000 - 13500 - 2000) - (100,000 + 20,000) = $89,500.
Each sibling gets about $40,000 in proceeds (depending on fees and outstanding mortgage balance) and each sibling owes and pays taxes on $17,900 (1/5) of the capital gain. Capital gains tax is 15% for almost everyone so your sib's different tax brackets may not make a difference (and it's not your problem if it did).
On your personal tax return, you would enter the sale of an asset with 1/5 the basis and 1/5 the sales price, etc. What your sibs do is their problem.
No one's tax return will say "I sold a house for $225,000." Each return will say, "I sold an asset worth $45,000 and here is the cost basis to calculate my gain."
Make sure you document the adjusted cost basis in case of audit -- you need to be able to prove what your parents paid to buy the house and how much they or you spent in permanent improvements. Don't guess. If you are audited, the IRS will assign the lowest basis that can be proven.
What you do with the rest of the money after paying the capital gains tax (such as gifting it back to your parents) is up to you. But it's important to note that if you own the house and sell it, then you get the money, you are responsible for the taxes, and you get to decide how to spend that money. No one else has legal rights to it.
