By law you can prove that u paid the mortgage for the past 29 years. Assuming it was a 30 year loan, you are entitled to 29/30 of the whole thing. The remainder (1/30) can therefore be divided amongst the two of you as you agree. The fore she is entitled to roughly 1/60 of the current market value of the house once sold. You can prove your investment through a receipt of payment (your returned checks) therefore it was you who took the risk of investment and are entitled to a return, if any, on that investment. If anything you are entitled to what you invested plus 1/2 of the profit made and even that is a generous gift to your sister.

As for right or wrong.... Your sister is a despicable person. Sorry to say that, but to try to seize one half of your investment following such a tragic event like the passing of your mother is horrible. She, as well as you entire family will be in my prayers

Keep on chuckin'