Please Help, I need your legal advise.

Hello everyone, I need your legal advise. My Mother passed away last 2003, My eldest Brother last 2006, my father last 2008 & my sister last October 2, 2010. We are five in the family & I'm the only one living. We have a family home at Sta. Mesa, but I wasn't able to fix the necessary documentations, extrajudicial proceedings, land tax, transfer tax, etc. My parents bought a house beside our family home for my brother & his wife, representing their share, while the family home will be divided to me & my sister. When my brother died, in less than a year, my sister-in-law sold the house and almost all the assets my brother left with her. My sister-in-law is living together with her lover. Now she is claiming that she should also have a share on our family home. Before my sister died, she was able to convince her to make a will in favor of his 14 year old son. My question is, will I have a chance in court if I can discredit her personality? And that she orchestrated the will stating that my sister is doing it without anyone coercing her, in her free will and in hand writing to attest authenticity? How in the world a sick person can think of such things, writing it last August, wherein she's suffering from stage 4 cancer? Obviously, there's a lawyer behind. Everyone knows that she has a lover & all they want is money. I just want to protect my interest and that of our family home. She will surely sell the part that she's getting because she's claiming that his son is entitled & the will states so.

I know I'm on the loosing end because our law states so & she has a copy of the will. Just want to know my chances, thanks all.