Future Children

So... This may seem petty, but I'm looking for maybe personal advice. I am not pregnant, but I am engaged and probably will try to start a family within the next two years or so. But both my and my fiancé's mother passed away unexpectedly and tragically. It's been detrimental to us, and honestly we helped each other through the hard times. 
Anyway, I always plan ahead because I want to be the best mom possible, but it kills me thinking my children will never have a grandma. And what do I say the day our children ask us "who's your mommy?"
My mother passed away from an accidental oxycodone overdose the day she had a surgery. My fiancé's mother liver basically exploded inside her from how he described it, she was a diabetic and an alcoholic. 
How would I answer my future children's questions without scaring them, but also being truthful? Idk why but this has been so heavy on my mind lately.