Her older sibling just announced her death Friday ... with Mia saying on Instagram that Tisa passed unexpectedly just a couple days prior on Wednesday, "apparently in her sleep." She adds, "If there is a Heaven, undoubtedly my beautiful sister Tisa is being welcomed there. She was the best of us - i have never met a more generous and loving person."

Mia goes on to say, the family at large is going through a hard time in light of this sad news.

Tisa used to be in showbiz back in the 1970s, but didn't nearly work as much as Mia -- and she certainly wasn't as famous either. Tisa was a teenager when she started acting in Hollywood, making her film debut way back in 1970 in "Homer," a hippie counterculture flick.

Overall, Tisa had 15 acting credits ... including a starring role in the 1979 horror film, "Zombie." She also starred in 1978's "Fingers" and made an appearance in the 1979 Woody Allen picture "Manhattan" ... right before Woody began his relationship with Mia.

While Mia was blowing up in her career way back when, she was actually pretty close to Tisa -- there's photos of them hobnobbing together in and around Hollywood as young, aspiring actresses.

Tisa quit Hollywood in 1980 and went to Vermont to be a nurse. She was 72.