I don’t think this is a post on which the voting buttons are helpful.
OP, you are perfectly entitled to your feelings, difficult though they are. It’s not unreasonable to feel upset that your DC can’t have a ‘normal’ relationship with their grandmother at the moment, because someone else needs to be her priority at the moment. It is natural to feel jealous, and inconvenienced and side-lined.
BUT
You are an adult, and the foster child is a little boy. A little boy who has suffered great trauma.
Your mum is in an unenviable situation and I’m sure she’d rather things could be ‘normal’ but she made a commitment through her work to care for this boy and keep him safe and give him stability.
Fast forward to July and my mum agrees that the foster child can not be in a household with younger children as he needs to be in an environment where he is the only child, and more specifically that he cannot be in a long term setting with younger boys due to jealousy. So she had confirmed our original concerns but he was still to remain. At this point, I wonder why she would still keep the child in this setting as it means we can never really function as a normal family.
Your mum’s ‘setting’ IS an environment where he’s the only child. Your family doesn’t live with your mum. You are a normal family.
It feels like my mum has chosen the foster child over her grandchildren. Both myself and my husband feel the same way so for our own sanity we stopped the visits.
You’ve prioritised your feelings over your mum’s and your own children’s feelings.
You need to deal with your anger. Perhaps you feel your mum has always prioritised other children over you if she’s been a foster care for so long. But you need to examine those feelings and get help for them.
Insisting your mum basically ditch her responsibility to this traumatised child in order to be the kind of grandparent you want her to be is not reasonable, kind or loving.