Yes, you can hire an attorney on his behalf and that attorney will be provided with all of the documentation involved.
At this time, has he actually been committed as an SVP? Or has he just been sent to Atascadero for the required psychiatric evaluations?
The state has to file a formal petition asking for the indeterminate SVP commitment and your brother-in-law is entitled to a trial on that issue and the burden of proof, just as in a criminal
case, is beyond a reasonable doubt. So, unless there is significant reason to believe that he is a danger to others, an indeterminate commitment cannot be ordered.
Whether or not the attorney representing your brother-in-law can share with you whatever information he gets is up to your brother-in-law. All such information would be subject to attorney-client privilege, which means that the lawyer cannot share it with any third part except with the specific consent of the client.
"Groping" would normally be a misdemeanor
violation of Penal Code section 243.4, but can if particularly egregious, be charged as a felony and be punishable by up to 4 years in state prison. Do you know the exact code section he was convicted of? And what was his original sentence?
Also, a conviction of 243.4 would not usually trigger an effort by the prosecution to obtain an SVP commitment. Which suggests that your surmise that you don't have the full picture may in fact be the case.