Lynn,
This is a tricky situation but I believe that giving up a 2.75% interest rate is challenging to give up as we may not see rates that low for a long time. It really depends on the purpose of the $250k to help your sister.
One option is that you could do a HELOC on the home now to get her the immediate $250k. HELOC rate's can hover between 7-10 % pending underwriting so you'd be locked in temporarily with a payment. However, rates could potentially drop in the next 12 months. If rates were to drop, you can now feel confident in selling your home, paying off the HELOC and getting a new downsized home at a lower rate than the current market. Also, this would give more time thus putting you in Long Term Capital Gains Exemption if you've lived in the home 2 of the last 5 years. You might also find that you have the ability to refinance in 6-12 months as that gives you more time to save cash and also watch the market. The only disadvantage is that you would pay the HELOC payment to buy time to see what happens.
I'm happy to discuss all options if you have questions about HELOC's.
-Connor