Executor/Beneficiary

My husband and his sister have been named as joint executors and joint beneficiaries in their mothers will. His sister applied for probate but has been in no hurry as she only works very minimum hours and is in receipt of many state benefits and has realised that it will effect their amount of benefits when the house sells. She has already stalled the probate documents by 2 months. The solicitor dealing with the application had not let us know she had hung into them. She has also attempted to apply for a mortgage and offered a sum for the property which is £75,000 below its valuation. My husband refused and she has since not been communicating.
Her and her husband has used the converted outbuilding for very many years and the double garage too for storage for free. The outbuilding can not be marketed very well ( it’s a converted lower part of windmill) as it is stacked from floors to walls in their ex business and household belongings. No internal photos can therefore be taken. We are getting no viewings hardly 5 months on despite it being a very unique property. My husband has asked if she could remove their belongings that are in the mill but she’s not communicating or doing anything. We realise she has a conflict of interest and we also foresee we are going to continue to have problems with her with regards to sale too.
My questions are is she allowed to leave the property full of her possessions as a beneficiary even though it is so full the outbuilding can not be viewed. What can be done if she refuses to take any offer on the property please?