Math to answer your other question (above - sorry was out at shop and screen had not refreshed), they can "fight" it and insist on playing a full role in the administration. However, you as executors are all equal and they cannot get the Grant over and above your brother. They cannot insist on running the administration either.
Practically though, you could try and agree a compromise. Ask them to renounce, but still submit the papers for the Grant on your behalf (OK this will still result in a charge) and you will naturally instruct them for the sale of the house etc etc.
Incidentally, if it is not a large estate and they think there might be trouble, many will drop it like a hot potato.......