There's quite a bit of information missing from your question:
Did your father die in the UK or in the Netherlands?
Did he leave a will or did he die intestate?
If he left a will, who did he name as executor?
Those questions are important because they define the content of the authorisation that the Dutch solicitor is seeking (which, obviously, the UK solicitor needs to understand).
If, for example, your father left a will naming you as executor, the document would need to give consent for your sister (and her solicitor) to act, on your behalf, in that role.
If, on the other hand, your father died intestate in the Netherlands, and Dutch law requires your consent before your sister can carry out the local equivalent of 'seeking a grant of letters of administration', the consent document might need to be worded differently.
Basically (to cover all of the possible quirks that might exist in Dutch law), you need to ask the solicitor to draw up a general document stating that you consent to your sister acting on your behalf in all matters relating to the distribution of your late father's estate. (That's basically what you wrote in your question. You've explained it to us, so it can't be any harder to explain it to an expert in such matters!).
Note that the UK solicitor will probably need you to prove your identity. Take your passport with you when you visit the solicitor's office, along with a couple of utility bills..
Chris