If they can both admit there is a problem in their relationship, and can agree they need some outside help, in whatever form that may be, then they will have made a wonderful achievement at their meal.
Their problems have not occured overnight, and one meal will not end them. I hope your sister will accept that the problems are not one sided, but belong to both parties, and will not nagg at him about the minor things, such as showers, unfinished jobs etc, or threaten to leave him if he does not do as she says in the long term. This will only make him defensive and unwilling to talk, he will feel 'OMG here we go again...' He may also have things about her he is not happy with, things are rarely one sided. Leave the baggage to be cleared out later, they need to simply agree that they both want the marriage to work and seek help. Then, enjoy the remainder of the meal, being together without the kids, getting a bit of the 'old days' back. Could you, or another family member, have the kids overnight? Sometimes, just being physically close with no distractions can work wonders.
She is sure she still loves him, probably the old OH not this one!She may be suprised to find that is also how he may be feeling about her too....
She is also planning on decorating over the weekend and try and cheer things up in the houseWhy? If the weather is good, get her to take them all out for a family day out......seaside, picnic in the woods, anything just get out and spend some time together in the fresh air away from the house. Clear the head. if he won't go, then go anyway.......its amazing the way getting out of the house can make all the difference. Decorate in the week....or, you have your sisters kids, let them have a weekend away?
To be honest, it sounds a bit as if your sister is fed up with life anyway, maybe her life is stuck in a rut? Does she work? Does she feel trapped at home with the kids and no life? Perhaps if she made some changes to her lifestyle she may become more like she was too. Its something we can all fall into, the trap of everyday being the same. Getting into selfish habits, its life. They can do this together, change the way they live, even if it means having different interests it gives them something new to talk about.
I really feel for them, and the kids, and I hope for all their sakes they can make this work.