I suggest that your daughter is not thru mourning the loss of her baby sister and would focus on helping her do that. When she talks about adopting I'd respond with a comment about missing the baby and how an adoption will not replace that baby. I'd talk often and for several or more minutes at a time about the lost baby. Focus on what she's lost instead of the adoption. If this were happening for me I'd get professional help.
I'd not discuss a possible adoption at all. Be direct. We are not going to adopt a baby. Yes, you may some day but adoption is not definite. You don't know enough about adoption and your own feelings and circumstances to know for sure that you will adopt. Your daughter is too young to be involved in the process that would lead to or away from an adoption. Therefore adoption should not be a possibility for her. She's too young.
I know pregnancy must be a painful subject for you but let me draw this comparison. If you were hoping to get pregnant you would not tell your daughter about those plans. You would not include her in the monthly disappointment if a period or talk to her about fertility treatments. Adoption is a similar process.
I understand that considering adoption is a way to soften the pain of your loss but in the long run, as you're experiencing, it causes other emotional issues. If there will be no adoption possibility in your daughter's world she will eventually accept that there is nothing that she can do to make it happen.
It may sound cold but I think it's important to remove the possibility of adoption from your daughter's mind. And then to allow her to mourn the loss of the baby and the loss of the idea of a replacement. She, and possibly you and your husband, have not felt the deep pain of losing your baby because you've softened the pain with the possibility of adopting another baby. I'm not sure I've adequately explained this concept or that it will helpful for you. I'm presenting it as a different way of looking at the reason for your daughter's focus on the adoption.
I suggest telling her that you are not going to adopt at this time and repeat that over and over without going into any additional information. Answer every question with, "we're not going to adopt."
Then I suggest that you not talk at length about what it would be like if your baby hadn't died. When she imagines what it would be like I'd respond with "you are very sad that baby didn't do that. Baby is gone. We are very sad." Focus on the sad feelings and letting go of the dream that if baby had lived this would happen. Let her talk about her imaginings but don't encourage them. Bring her back to baby is gone and we are very sad. Do this to the point that she cries and comfort her if crying is what she needs to do. Has she cried about losing her little sister? At one she wasn't aware. She's aware now and must grieve that loss.
You have my empathy. The loss of a baby and the loss of another pregnancy is so very painful. It's understandable to want to soften the pain for your 6 year old but that isn't the best way to help her move ahead.
I suggest that one reason she tells you she'll share her toys is that you or others have told her that she'll have to share her toys in an effort to impress on her the reality of having a sibling. She's not had a sibling. Having to share her toys with a sibling has not meaning for her. She just has to convince you that she will share and then everything will be alright.
This is one reason I suggest that talking with her about why you aren't adopting now only makes the issue more complicated and seem like something she might be able to influence if she could only find the right way of doing it. If there is to be no adoption then she has not control. This may make her angry. As odd as it sounds anger is a more helpful emotion than longing for something that you cannot have. Anger is the first stage of grief.
A counselor specializing in children and grief issues could be of great help to you.