I am also a nudism loving person i also do the same way as you have mentioned Sheetal you have to be careful ,
There is no need to invite anybody else in between you and your husband ,Never think the brother in law will be a controlled guy .Just avoid it Why I am giving such an answer is you existing nudism built up and affections will be destroyed and there will be lot of issues ,That guy will come out to have sex with you also which you will not like I am sure .I have a belief according to me Sheetal you are a highly understanding women and your husband is lucky to have you as his wife see from hiss point of view when somebody else join in nudism with you both perhaps no normal husband will allow ,even if allowed then just avoid it and say its not possible.
I live with my partner 26 and her aunt who is 36 and i am a male in my age of 34 ,Me and my partner always stayed nude as much as possible in a week while during the first days we did in the first floor while aunt was bellow here what happened aunt also wanted to enjoy nudism with us since she is not related to me but a distant relation of my partner , my partner she told me very casually that see aunt has never got married and she way always living alone let her at least enjoy a bonndagless life in nudism .I agreed During first few days things went well she also stayed nude slowly our talks all went on having sex aunt started helping my partner during sex ,my partner she was a broadminded one day it happened I had sex with her aunt while we were all nude my partner initiated this ,after then its 4 years that I am in sex relation with both of them ,,,,here 2 females and me now imagine if it was two men and my partner all sort of problems can come into picture its very difficult..
For me I did not liked aunt to join us in the beginning but it happened in a wrong climax but there is noway to get out of this .I am not complaining my situation since i am enjoying ,you have to study your husbands need and never try to change him into new solutions since it will be never ending with you .
So Sheetal you remain in nudism with only your husband and enjoy life rather than bringing a new guy your mind will get no peace and peace less mind cannot enjoy proper sex.
I wanted to make you understand how things work in nudism ,Since yu all stay in one house it is major drawback in bondages while nudism makes people bondage less .
Another example one of my office staff she is 34 widow but a great writer she believes in nudism she organizes
nude cofee time at her home were she will make a point that all people coming stay not more than even numbers come maximum 6 people and me ,here our intention is not sex when we assemble we at her house we spend 3 to 4 hours no other group is allowed to join us also no couples are aloud ,we all get cloth-less and enjoy a very freedom stated mind of thinking I love nudism but it should not be in the stake of sex .
We never pollute nudism with sex other than our partner so everybody will not have that feeling with nudism so just avoid him stay with your husband enjoy life of freedom with him only.
You have to take nudism into different dimensions you are practicing it with your husband limit to this much just ask your brother in law to stay away totally since he has mentioned to join you this is the only solution .
Its the matter of peace in life and self esteem in freedom of thinking dont mix sex with life.
According to me and your pattern of question you have lot of career plans go for them make your husband a happy man by giving the most out of you to him .
Also note benefits of nudism are there but i feel its not applicable to your family since the question you raised is the answer itself I just elaborated with few examples so that you can anchor properly.
Advantages of nudism I got out of life
Partners remain question less in mind quarreling couples will soon live the habit of quarreling,its not about the beauty of body that matters in nudism its the freedom of thinking which grows between partners ,level of excitement increase between couples ,couples body automatically starts showing signs of new development to self healing , ,arguing mentality of people reduce ,all mind blocks vanish ,
practicing nudism makes health related issues and problems away,Body skin glow ,people become more beautiful in look,people start appreciating there body ,people start respecting their body..a lot more
anyway think wise and make sure a serious trouble is not welcomed in life by inviting your BIL in nudism .