Whether or not her existing income is enough depends on expectations.
For some people it will be loads, others will struggle, but if her rent and council tax is covered, she still has quite a bit left over after essential food and bills. I think she will also get the winter fuel allowance, but maybe they already had that or it might need reclaiming if it was in your dad's name?
If she gets £177 pw, that's actually £770 pm, which is a little more than you say. It's nice that you and your sister want to help her, but don't do this to detriment of your own finances, especially if your DM is one of those people who spend loads of money on crap she doesn't need. On the income she has, she's enough to put the heating on, buy food and have a decent amount for discretionary spending.
If she's happy to downsize, that will help to keep the heating bills affordable, and the property easier to manage as she gets older. You never know, some councils offer financial incentives to underoccupied tenants to free up larger properties for families, so something to look into.
My DM is in similar circumstances. Her income is around £900 pm so slightly more but pays pay council tax, she's a homeowner, so no rent to pay anyway. I don't think she gets pension credit, as she gets most of the state pension - she was a little short on NI contributions, but gets nearly all of it, plus she gets a small widow's pension. She has plenty of money to be honest, but modest tastes and doesn't spend extravagantly. But she does buy things for the house, gifts for grandchildren, does her food shopping in M&S and Sainsburys and has pets.