Thanks for your reply.
You can obtain a copy of the Will from Leeds Probate Registry, IF Probate has been granted.
Probate is required if a deceased left over £5,000 or owned a property.
Here is the link to do the necessary search- https://www.gov.uk/wills-probate-inheritance/searching-for-probate-records
On a side point- depending on how your Mother was holding the property with your sister and brother in law, may mean the whole of the property has passed to your sister and brother in law upon your Mother's death. This will be the case if all 3 of them were holding the property as Joint Tenants.
The other way that property can be held is as "Tenants in common" which means each of them owned a separate share in the property. If the property was being held in this manner, there will be a Restriction in the Proprietorship Register saying "no disposition by a sole proprietor is permitted otherwise by an order of the COurt" or words to that effect.
If there is no such Restriction, then the parties were holding the property as Joint tenants, which means that upon your Mother's death, the property passes automatically to your sister and brother in law (like a joint bank account) and your Mother's share of the proeprty does NOT form part of her Estate.
I hope this assists and sets out the legal position.
If I have helped, I would be grateful if you could rate my answer.