You are a wise, caring sister.  YES, he WILL do it again!  Here in my state, you can still press charges.  There is no time limit.  Go to the Department of Social Services and tell them you want to report sexual abuse (it would be better if your sister did this).  Don't be afraid to go, and STOP taking Mom's advice (shame on her for making you girls keep quiet about it).  They may not be able to convict him now, but at least people will know what he did, and others will beware of him.  Now, don't be afraid.  I will be praying for you and your sister to stand strong.