I know someone this applied to this year just gone and I remember the discussion before the results came out and after a quick check here it is from the admissions criteria. Unless it changes you are guaranteed places for both:
It is the policy of the Governing Body to admit both (or all) sisters or brothers in the case of
multiple births, provided that at least one of the siblings gains a place at one of the schools
under any criterion and that the school is deemed appropriate for the educational needs of the
additional child(ren). The additional place(s) will be counted as sibling admission.