Out Of The Loop

Ok, this really upsets me and I'm not really 100% sure why.
My sister is in rehab, for a six week program that was mandatory following early release from a federal prison.  She was to get out on Oct.2nd. 
I was at my mums the other day and April came into conversation.  I'm travelling to the city she is in currently for some other reason.  I asked mum if it would be possible to take April out on a day pass on the 27th, as it's her birthday.  Well, mum informed me that April isn't allowed out on day passes during the week, only the weekend.  We are going up there Friday night until Monday night but the weekend is booked.  But them mum told me it didn't really matter because April was back in prison.
I asked her for what and when did this happen?  She told me it happened because during a time of duress for April another woman in the rehab program hugged her, and April hugged back and there is a no contact rule in this rehab.  They were both sent to jail.  Well, April is in prison until the beginning of October upon which she will return to the rehab and start the six week program all over again.  My mum told me this happened a while ago.
The kicker, she said she hadn't bothered to tell me because she didn't think I cared.  My mum isn't usually so insensitive and cruel.  It really bugged me.  It seems because while April was an active addict(I didn't enable her AT ALL) becuase I wasn't there for her the way she wanted me to be it must mean I didn't care at all.
I've been attempting to glean a relationship with my big sister from all this rubble and it's been slow going.  Then to find out I'm not told what's going on with her because no one thinks I care?  Piss on them!