Hello again & thanks for the extra info.
The question of which parent the children are to live with when their parents separate depends on what is in the best interests of the children. If the parents can agree between themselves, then there is no need for a court order. If they don't agree, then either parent can apply to court for a child arrangements order, which will set out who the children are to live with & how much the other parent is to see the children. The court must consider a range of factors, such as the children's educational, emotional, physical needs, whether there's a risk of harm with either parent, and the capacity of each parent. The children's wishes and feeings also play a part, and the older the children are, the more weight are given to these. The views of a 15 year old will almost certainly be decisive, unless there are strong reasons why she/he should not live with the parent of their choice.The views of an 11 year old are not usually given nearly so much weight - but the court is VERY reluctant to split up children, so most likely the children would be kept together - unless there was strong reasons why that should not happen.
Generally, the court will order that the children remain with the parent that was their main carer before the parents split up - as long as the children are happy with that parent, safe and properly cared for by them.
From your question, although you do not spell it out, you seem to be implying that the mother may be leaving the home, leaving the children behind. If she has been the main carer until now, and a good mother, that would not in principle jeoprdise her wish to have the children move to live with her at a later date - but it might depend on how long she was away from the children, whether the children's wishes changed in the meantime, how far away from the children's current home she was plannning to live - the courts are very keen to avoid disruption to children if it can be avoided eg would the children have to change schools?
Regarding the financial side of the marriage breakdown, whichever parent the children live with, can claim child support from the other (actually child support can be claimed on separation, regardless of whether the couple divorce.) Once a divorce petition has been filed at court, either party can make a claim for a fair share of the matrimonial assets, if the couple cannot agree between themselves. If one party has a significantly lower income than the other and/or provides a home for dependent children, then they can claim a more than 50% share.
Your daughter needs to get some face-to-face legal advice from a specialist family law solicitor. Here's where to find one:
I hope this helps and I wish you the best of luck.
Thanks and best wishes...