If you feel your sister is acting improperly and not moving as quickly as she should, raise it with her and ask for an account showing progress to date. You can then discuss this with her. She may agree to behave differently or to stand down as executor.
If she refuses to change you may be able to remove her as executor for incompetence or dishonesty. If there has been mismanagement — such as failing to distribute the estate correctly, or not paying off debts or tax due — she could even find herself personally liable to compensate those affected.
If it went to court you would have to show serious misbehaviour before the court would remove her. Stealing from the estate could well be enough, if it was major. So could failing to keep accounts or mismanaging the estate — for example, allowing it to reduce in value through inaction. However, a court is unlikely to remove an executor simply for being slow if this has done no real damage.
It is worth speaking to a solicitor at the outset so you know exactly what your rights are if she won’t co-operate.