I'm going to restate your situation for clarity (gathered from the question and comments). Your sister-in-law seems to be basically saying this to you:
I know you and my late husband agreed to a 75/25 split when you purchased the property, and I know you both continued with that split for 20 years, and I know you filed your federal tax returns with that split too. But, guess what? You messed up. Since you didn't put the 75/25 split on the deed I'm technically entitled to 50% of the property instead of just 25%. So, now you must give me what's (un)rightfully mine.
This is unfortunate, as it means your relationship is tainted and probably not salvageable. I think you have every right to respond this way:
I'm not giving up 25% ownership just because you want it. If you want to sue me and take me to court, go ahead. I'll bring all of the documents from the sale and our tax returns which clearly show the 75/25 split, and let the judge decide if you should be given 50%.
My guess is when faced with this option she'll stop grasping at straws. Of course, if she does sue you, you'll probably need an attorney if the amount of money at stake is worth the fight.