I don't understand what you think is unfair, or maybe I don't understand what the siblings want to do.
You have to think of this as a business with 5 shares of stock and you each own one share. Or as if your parents gave you 5 gold coins, 1 each. You each own an equal share as an asset. In fact, you could sell your share to a stranger, right now, without their permission probably. You own an asset that happens to be 1/5 of a house.
There is no tax to pay before the sale. No single tax return will show sale of a $225,000 house and a large tax bill. If you are co-owners then each co-owner will report the sale of a $45,000 asset (1/5 share of the house) on their tax returns. If the overall cost basis is $68,000, then the cost basis of each share is $13,600, and the taxable gain for each owner (each share) will be about $31,400 ($45,000 asset price minus $13,600 cost basis). There is no way to set aside money in advance since there is no single "sale" with a large tax bill. There are 5 small sales with 5 small tax bills.
The long term capital gains tax will be 15% for most people, or $4710. There's just no way to pay it in advance, or pay it "out of the sale" in one lump sum. The sale will have to reported on each co-owner's tax return as the sale of a $45,000 asset resulting in $4710 of tax.
(This is federal taxes. There will probably also be 5% or so state tax, when the income shows up on your state tax return.)
If the house is owned free and clear, each co-owner should expect to get $45,000 in sales proceeds (minus commission and fees) and owe $4710 in tax so each owner should net about $39,000 after tax. What you do with that money later (such as, give some back to your parents) is up to you.
And, if due to some scheme, you never got any of the cash from the sale, you would still have to report the sale of a $45,000 asset and owe capital gains tax of $4710.
It sounds like your parents made a terrible decision 15 years ago to give the house to their kids, because now they want to convert the house to cash for themselves -- but they don't own it any more. It's not their house and it's not their money. The children may have a moral obligation to contribute to their parents' well-being but it is not a legal obligation.
Now, maybe what you need to do is sell the house, each child takes $45,000 in proceeds, pays $4710 in capital gains tax, and then puts $10,000 in a trust account for your parents as a gift. That would give the parents $50,000 of spending money, each co-owner pays their own capital gains tax, and each co-owner have about $29,000 left over for whatever.
Important information!
If you are equal co-owners, then the other siblings CAN'T SELL unless you agree and sign the papers, and you don't have to agree to anything you don't like or don't understand. They could take you to court to force a sale, but that would cost way more than settling with you.
Or, maybe you should have the other siblings buy your share before they sell the house and do whatever. Buy you out now and leave you out of their future plans. You can sell your share for any price you think is fair, with the understanding that you still have to pay capital gains on the sale of your share (Sale price minus $13,600 cost basis).
Regardless of what else happens, YOU need your own lawyer at this point I think. If your sibs keep it simple, it should cost less than $1000 to have a lawyer help you sell your share of the house and protect your rights. If your sibs make it complicated it will cost more of course, but they will then be having to pay their own lawyers.
And for you, it's going to be better to pay $1000-2000 to keep $39,000, than to not understand your rights and end up feeling cheated, or being actually cheated.
