difficult behaviour

ITA with all above. I am also aware that we ask advice from people whose opinions we want to hear.
So although in principle, the "guiding to solutions" & "parenting groups" are excellent advice, I think DiL wants your take.
You obviously have at least 2 sons, so I think she wants to hear tales of their childhood, and how you coped with their rivalry.
I also assume that she thinks fairly highly of at least one of your sons (!) so may want the reassurance that her son will grow up OK.
You can share your own experience & ideas, whilst recognising that their family circumstances are different, and that she may find things difficult that you found easy (and vice-versa).
A useful question is to ask what she would like to be different / do differently and then ask how you can help with that.

If she has a local Sure Start Centre they may run parenting classes, everyone is welcome the staff are positive and helpful and the emphasis is all on positive re-enforcement and having fun with your children.
Some of them are closing with the "cuts". They are free.

I think you're right about the attention seeking. I've also looked at a couple of websites and the suggestions seem to be along the lines 'disapprove of the behaviour, but not the child' and this is what DIL finds hard. She yells, he sulks, and they are in danger of getting into a downward spiral.
Now I just have to practice what to say, and how to say it!
Thanks for feedback.

It sounds to me as if this is at least partly attention-seeking behaviour. You don't say what the age gap is between your grandsons countrygran but it could be that the elder one is expected to be more grown-up in his behaviour than he feels ready for and resents the fact that the younger one always seems to get away with stuff. It might be worth suggesting – tactfully, of course – that your daughter-in-law spends a bit of time exclusively with her older son. It doesn't have to be a lot. Maybe you could offer to do something else with the younger one outside the home while mum and the older one do something that he is interested in. Equally, she could just let him lead the way – he wants to read a book, he wants to paint, he wants to build a rocket ship. It doesn't really matter what as long as they are doing something together. I know my elder grandson, who is a fairly nice boy, does get fed up with being expected to keep an eye on the younger ones and generally be more responsible.

countrygran

Thanks, I do try this with her. The other DIL asks for my opinion, I give it, and she's happy to try out my suggestions. I guess this DIL is less sure of herself, and asking for advice/help implies a failure on her part (in her own eyes). I do hope to try helping in the way you suggest.

You could try the Socratic Method:

http://en.wikipedia.org/wiki/Socratic_method

This just involves talking to her, asking questions that sort of lead her to her own answers to her problem. If she thinks of the solution herself in this way, you won't be the bad guy.

Of course, the answer she needs to find is to give the children boundaries, enforce decent behaviour by refusing accept bad behaviour form either child, and realise that she is the parent, she is the boss, she is not their servant or entertainment director.