If your mother hadn't left a will, and your aunt was therefore applying for Letters of Administration rather than probate, nobody would be entitled to touch the estate until the Grant of Representation had been obtained.
However where a will exists, executors seeking probate can lawfully start 'tidying things up' prior to a Grant being obtained. If all of the executors are happy for the monies to be paid into your mother's account, and for her to pay off any bills and to pay for the funeral, there should be no problems.
Chris