I have found over the many years, you must be frank with them.
You say the one same line, each and every time...and if they don't get it you just do not talk about it.You change the topic.
I would always say''That is something I just can not talk about-and I know you will understand''.
That has always stopped them....end of topic.
If you say the same thing, for each time they ''Try''.....they get the message, very quickly.
You must always say it with a firm, but loving manner.
Emails are much more easy, the written word is powerful..
I have had to deal with family who are in the Law Industry, and it was often like being on the 'Trial''!
But you have to just be firm...and very polite.