Telling Baby one day

So my sister-in-law (husbands sister) passed away Tuesday morning after a 5 year long battle of metastatic breast cancer she was a smoker and continued after her 2014 diagnosis. He was very close to her so he was very upset after her passing he was 1/6 kids. My daughter is now almost 18 months old. She has Down syndrome and so does my S.I.L daughter she’s 15 years old. But I don’t know if she’ll really remember her and if at all she’ll only remember her as being sick and very thin. And I’m afraid if she asks my husband or I one day who she is since we have a picture of her last birthday this year on our fridge which we’ll keep forever I’m afraid that he’ll get upset since it’s hard for the name to come up without getting upset. But I’ll admit the last few days of her life she was semi-conscious at home in bed by hospice In morphine and on oxygen. We visited her a couple weeks ago when she could still talk and walk and he wanted to ask her then about what to tell Scarlett (our daughter) one day, since she had to explain to her own Down syndrome daughter about her dying. But we had left as she was tired from her trip to the hospital over night on June 19th. But we never got to hear her tell us as she was in a semi-Conscious state and couldn’t respond to us. It was very hard seeing her as her body was slowly starting to shut down. Any parents of DS kids who’ve had to explain to them about a family member who died of cancer?