Actually federal capital gains tax rates are currently 15%, but that may be increasing. Speak with your tax advisor for specifics.
There are also state capital gains taxes, which I believe are 7% in Connecticut, but I'm not positive of this rate.
Is the vacation home used as an investment property? Is it rented out, or is it simply a vacation home for you and your sister? If it meets the guidelines for an investment property, then you may be eligible for a Starker 1031 Exchange, where you would sell your share of the home to your sister, and then immediately use those funds to purchase another investment property of greater or equal value. This would postpone the capital gains taxes until you sell the new property. There are specific criteria and timeframes involved in a 1031 Exchange, and you would need to hire an Exchange company to handle the funds - all funds must go into the new property and not to you.
Another option to consider would be to move into the home for two years, use it as your primary residence, and then you can sell it while taking the $250,000 capital gains exclusion . Again, you need to speak with your tax advisor regarding your specific situation to determine what you qualify for and what options apply to you.