Nobody can predict what a Family Court will decide so it’s a bit of a waste of keyboard wear and tear to suggest it.
However, Family Courts will take into consideration the time children spend with each parent and their siblings (and that includes half-siblings).
They also understand the issue of ‘quality time’ (I hate the term but it is descriptive) by which is meant the time outside the routine of get up, breakfast, school, homework, tea, bath, bed. This is why children frequently spend alternate weekends and half the school holidays with the non-resident parent. The Courts also encourage children spend a weekday/night or two with the non-resident parent as that gives the non-resident parent a chance to engage with school work, other ‘boring’ routines and allows the child(ren) more time with half-siblings.
It is a fact of life that as so many parents work there will be occasions when a parent’s ‘new’ partner/spouse will have the child(ren) in their care. It’s what happens in families.
Children do need stability but at the age of 11 they are aware that different households can have different 'rules' or routines.
I would still suggest the OP speaks to a Family Court Solicitor is she can’t come to an amicable solution.
One thing the Family Courts will NOT do is take into consideration any Child Maintenance payments.