That’s one of the problems with foster care, the foster child gets a view of a very different lifestyle, benefits from
that to some extent while also maintaining relationships with a birth family where things may be much more chaotic. Part of the job as a foster care is to help manage that gap. You honestly can’t see why her telling her, by very definition, struggling mum how lovely her life is and her wonderful plans for the future - a life mum will never be able to give her siblings - would bring about an adverse reaction?
It’s not your job to call out her mum, it’s your job to support continued relationships with her family of origin by helping her understand the reasons why her mum can’t care for her in a positive, affirming way, no matter your personal opinion of mum. It’s a balance all adopters need to strike - being honest about the reasons for being in care, and ultimately being adopted, while leaving that door open should the child want a relationship with their family of origin now or in future.
She has a right to a relationship with her birth family, which in time may become very important to her. Even if you do go on to adopt her that right remains, particularly given her age. Of course as a young teenager she can see the differences in what you can offer her v what her mum can, and doesn’t want to return to her family, but that doesn’t mean cutting that legal bond is in her best interests in the longer term. You need to really think about how and what you communicate about her mum, because “look at all the lovely things I can give you that your mum can’t” may bite you on the arse when she gains maturity and understanding.
As a foster carer you have a duty to raise any concerns about contact with her social workers, because there are other children to consider here and they also need to be able to assess whether continued contact is in this child’s best interests. You seem to have lost sight of the wider responsibilities in your job, because fostering is a job, while also being much more.
Your fosterling deserves to have a sense of security about her future, which doesn’t necessarily mean adoption or a SGO, it could be that the current arrangement is secured through permanence planning without changing her legal standing with her mum. You clearly have a very privileged life, one which makes parenting much easier in lots of ways. Consider where her mum would be with your advantages, and how your parenting would be if you’d had her mums life experiences and adversity, and again think how it would be if you saw someone else giving your child everything you’d never be able to give them in a million years, and think about how to help your fosterling to straddle that gap.