New puppy or no?

Can someone that's been in this situation give me their input?

We have 3 dogs (2 acres of land so we have room). One is a 12 yr old ish rescue. She has breast cancer and is slowing down and likely on her last yr, and a 10 yr old Chihuahua who can handle her own. Our other lrg dog is 6 and has never been an only dog or without another big dog.

We have the option of getting a puppy right now. Part of me wants to so the new dog can have some of our older girls personality/temperament. And then part of me isn't sure if it would be wrong to do to her? I don't want to stress her out and make her go faster, but I also don't want our youngest pup to be without another lrg dog, I know he would love a dog to actually play with. Tia!