Invest into my parents home for higher percentage cut in will?

[Myers Crook]

I am 18 years old living at home and just dying to make an investment plan. Im an awesome saver ( 20k saved out of a 30k/yr job) and want an investment goal. My life is slowly all coming together, aswell as I have recieved a new job making over 50k a year and not sure what to do with it. ( assuming I save 40k/yr) My parents are struggling more financially, retirement for them is not in sight and theyre not getting any younger, theyre both 58yrs old. They have paid around $70k into a $170k mortgage over 20 years so far and have been constantly falling into credit card debit, renewing their mortage over and over.. its never ending. The idea in mind: Saving 100k in the next few years while living at home, paying off their mortage, leaving retirement a better possibilty for them as well as mentally relieving them from some of lifes stresses, and then ofcourse when they pass Ill get a higher percentage on the will and gain the appreciation. 100k ÷ 170k .. is around 59% Id recieve of the final selling price.. ( The house has been valued at 500k) And then the 70k would be split 50/50 between my sister and I. (Keep in mind the basement is not finished.. maybe should I invest a little for that, for a higher final selling price?) Im new to all of this and constantly trying to learn the game of investing. If possible, leave your opinion, if this is a wise decision or not and maybe what you'd do as an investor personally..?