Mistake in Family visitor visa form

Hello,

My wife's sister invited her parents last year on family visitor visa to UK. They applied through agency. Visa was sucesfully issued and they visited UK and then left within stipulated time.

Now my wife want to invite them this year but we realized that agency never included any of my details under "other close relatives in uk" section of the application form. We came to know when we asked parents to send us copy of their previous visa application form (which was submitted by agency)

How should we proceed with their visitor visa application - as we want to bring this error to home office's attention without risking their visitor visa application?

Any guidance will be much appreciated.

P.S. Both sisters have permanent resident granted in 2012 and are working professionals here in UK.

Thanks very much in advance