Can two members of the same family use the same firm of solicitors for different cases?

My sister used a firm of solicitors to ist her when her relationship broke down and access to their child had to be dealt with through the courts. A number of years later my mother went to the same firm who have isted her (I have had to sue my mother for money I spent on her house which she refused to pay back), written her will after a restraining order (for monies owing to me) where my sister is the sole beneficiary and then they wrote threatening letters on behalf of my sister claiming I was haring and defaming her. My mother has since died and the firm are now the Executors of her will which I know is probably the usual way. Can they both use the same firm as when I was looking for a solicitor I was asked if any family member had used the firms I was contacting and they all did a check