SSP, PIP, and part-time work - is UC obligatory?

My sister spoke to me earlier, and is very worried.

Her husband has been very ill for a while now, and had been working full time until recently, when his health took a real downturn, to the point that he is now off work, and on SSP, with the chance of an early retirement through the health problems. My sister works part-time - something she doesn't want to change.

Anyway, my BIL was told by a friend that he should try and apply for PIP, as his health is that bad and, last week, he went into their local job center to claim it, and came out, having been told he should also claim UC, and my sister should, too - neither he or my sister have ever claimed anything, as they've both worked all their lives, and they don't really want to have the UC, as my sister says they can just about manage on the SSP and her part-time work and, if the BIL gets PIP, they will be fine financially.

Apparently, this morning, she had a phone call from the job center, and the woman told her off as neither my sister or BIL had been writing in their journals? Neither of them knew what that was, as they hadn't been told about it, and so the woman has arranged for them both to call in the job center tomorrow to 'discuss' it!

My sister is now worried sick, as she's heard about the nightmare people have been through with UC, and she says she doesn't want to claim it, and doesn't understand why they told her hubby that they had to - is there anyone here who could give me any advice to give them before tomorrow, please?

Personally, I feel that, if they can manage financially without claiming UC, then they shouldn't be forced to, but I don't know if my BIL having SSP has anything to do with this, or not.

My sister is terrified they'll be forcing her to look for more work, when she doesn't want to do any more than she is right now

I only have this evening and early tomorrowto let her know where they stand on this, so would be grateful for any advise to give them?