HM, sorry if I sounded like I was having ago at you in my previous post, I wasn't. Looking back my post may have come across as if it was aimed at you which it wasn't!
The issue's in picture isn't just about the sow looking extremly under weight, it's the bad practice you can clearly see going on, and you can see all those pigs are paying the price. Just take a look at those piglets.
A sow and litter should not be mixed with a boar and weaners of different ages - its a recipe for disaster.
Hm interms of your sow, have you tried giving the piglets a crep feeder where mum cannot get too, (which always has feed for the young ones) then feed the sow away from the piglets to maximise the feed in take of the sow? I don't know if you do already, but I would stick her on a finisher diet, as high in protien as you can get your hands on.
I would also wean early around 6 weeks dependant on her condition at the time.