Unfortunately, without the additional information requested in my first/previous response, I will not be able to give you detailed advice to your question, but I will be able to give you a general answer, which I hope will be of assistance. If you are able to supply the additional information later, please do so and I will then come back to you with a more detailed answer.
The fact that the solicitor is the brother of the other party does not matter. Family can indeed use a relative if they are a lawyer.
They do however still have to act and behave in the same profession manner as they would representing any other client.
A legal letter may sometime come other as being threatening when I face it is not, it may be forceful and direct to the point, but that is not a threat.
If for example the latter states that if you do not do something etc legal action will be taken or reported, then that is not a threat, that is standard legal action.
If however direct non legal threats have been made, for example, “ if you do not leave the property we will drag you out” then that is indeed a direct threat. That is threatening violence and can be reported to the police. It can also be reported to the Legal ombudsman and the solicitors regulation authority. Links for these are attached below.
I hope this helps address the question you have raised. If I can assist further please come back to me.