I used those exact arguments for 'persuading' myself to leave my DDs father. However, looking back, the real issue for me was his attitude towards me and his attitude towards nurturing a family and the running of a household - there were a few other issues too. The issues that your DS state are perhaps symptomatic of a one-sided partnership.
Your sister is crying out for help, her DP needs to understand this, as do you - as her sister you can do a great deal to help.
I would advise that she begins by trying to right the imbalance. Perhaps start by remembering the original attraction...? Or perhaps something completely new? Perhaps you could take the children for a week whilst they get away somewhere? (in the hope that they could begin a new path with some more positive things to think/talk about), and I think you could try persuading other family members to step in and take some of the strain off her and her DP on a regular basis (at least one morning/afternoon on the same day every week)thereby giving the partnership the opportunity to flourish again. Splitting would be a last resort IMO, but that is only if I take your post on face value. I suspect that there may be more factors at play here that need dealing with, in which case I may well re-evaluate my opinion, but even so a last resort (unless there is violence/drink/drugs involved)
As for whether it is easier to soldier on or get rid...? An internet forum with all types contributing is not the best place for a definitive answer - but I can see that getting opinions may help your DS gather her thoughts and formulate a plan.
I left my DDs father, but there were other factors at play, so please don't take that as a guide! It has not been easy and at times I feel trapped in the house, isolated from the goings-on outside, even though I am not, and the middle of the night is awful. Then there is the disciplining, the relentless cycle of 'to-dos', the holidays away (camping and one child needing the loo in the middle of the night, flying and trying to board/check-in with baggage/children/...) I have had unwanted attention from men who have thought I simply must be desperate and up for anything, simply because I am alone, and have had strangers walk into my house drunk in the middle of the night (this seems to be a surprisingly common experience for single mothers!)I struggle under the weight that the survival of this family rests upon my shoulders alone. There is only one income coming into this household - one is not enough (unless you DS has already climbed the career ladder?)! I have to choose work to fit around my children, and am called upon first when there is an incident at school... Then there is the general view from society that all single parent families produce criminals... It WILL effect they way the school/Drs/neighbours etc. look upon your DS (quietly, but it will - we all hear the little throwaway comments about single parents - some people actually believe these comments). The list goes on! Being a single parent is very very very hard, and should not be taken lightly! However, it is not impossible to succeed. Nor is the whole world against single parents, an increasing number of the whole world single parents (unfortunately)and so this leads to a greater understanding about single-parenthood.
What I am saying is, its not just a case of lightening the load at home by cutting the amount of housework, becoming a single parent, for what ever reason, is not an easy option. If it is simply that your DS is struggling under the weight of bringing up a family then doing it alone will not help her. If, she and her DP have issues then they need repairing. Either way, she needs her family (mother/father/brothers/sisters/cousins...) to step in and take some of the strain.