I'm stuck!

I have a dog, Sierra. I've had her for 7 years now. I used to have her sister too, but she died 3 years ago on Thanksgiving. So, I've just got Sierra now. Sierra is not a friendly dog when it comes to other animals.

We have gotten a couple puppies here and there and never been able to keep them for various reasons, mostly being that she just refused to like them and I was afraid that she would hurt them.

We did get a puppy once that was similar to her and she did end up liking the dog a little bit. But I had to get rid of it because it seemed to be making Sierra sick. I had picked that pup up in the middle of the highway and didn't know it's history and Sierra was going through heart worm treatment and I was afraid the puppy was carrying a desease that would kill Sierra. Soon after we got the pup she started having bad bowel movements. So I had to get rid of her.

Anyways, I have a friend who's dog just had pups. I've loved the momma dog since I met her and I would LOVE to have one of her pups. But, I'm scared that Sierra won't like it or ever get used to it. I'm also scared that if we got the pup and kept it around for a while that the pup would end up hurting Sierra because it's going to be a way bigger, stronger dog. I just have so many mixed feelings. I really think that if she would give it a chance that she would LOVE the company, I just don't want to take ANY chance that it would be bad for her in any way.

So, I'm stuck. I don't know what to do. What would you do? I would appreciate any suggestions!
Thank you,
~Stephanie