Написать ответ на приглашение на языке к следующему обращению dearjohn, many thanks for your letter which came yesterday. it took a month. i am now sending another invitation as requested. i shall try and send this to you by personal courier in view of the postal problems. i wonder when your are thinking of coming to moscow . in some ways september would suit me best, as i have to be away most of the month and would like to have a «watchdog» in the house. but please arrange things as it suits you best. with best wishes. yours very sincerely,

Dear Luckyguy,

I was very glad to hear from you again. I have written a message to you quite recently and yesterday I got a letter from you .
I got your invitation to come to Moscow in September. That would be great! But unfortunately, I cannot come to  your place and have a fun with you and your friends. My older sister is going to come here with her husband and children. So I have to spend time with her kids (my cousins) and help my mother around the house. It is always so awful  when the house is full of guests even if they are your relatives.
I don't understand what you have written about "watchdog" but I think that you'll explain me this situation in your next letter. 
I wish you have a good holiday, relax and have fun.
That's for me today. I have to go to the swimming pool. 
Best regards to your family.
Write soon,
John