Do be aware that UK student finance is based on residency, not nationality. So, unless you can prove that you're only there temporarily (they tend to look at things like your employment contract i.e. are you on secondment, if you maintain a home in the UK, and where you spend your holidays) she will be classed as an EU / international student. This has big implications.
If she's classed as an EU student, she'll still get a student loan for fees (which will be £9000) but she won't get any maintenance costs paid for. In fairness, given your income, this only makes you about £3500 worse off than you would otherwise have been.
If she's classed as an international student, you'll have to pay international fees (anywhere from £15-35,000 per year, depending on uni and subject), and be eligible for no student finance.
Her being back in the UK for boarding school would mean that she was mainly here for the purposes of education, so she wouldn't re-establish UK fees status that way. It takes three years of being here for reasons that aren't mainly about education to be classed as a UK student.
You need to take account of this for both her and siblings before you start to even think about boarding school - there's no point getting an offer from Oxford if she can't afford to go.