Maybe you have to be a bit harder then (my eldest is 6 so I have no experience of this btw). How about stop doing his washing and/or ironing until he can do simple chores like carrying the clean clothes to peoples rooms - not a lot to ask in exchange for freshly laundered clothes.
Sounds to me as though this isn't just really about the dog? More like the whole running of the family? Have you been together long? Have things got worse or did they never do much around the house? Who did everything before you? Is it the SCs pet rather than yours? I think the family meeting sounds like a good idea but you must have DH onside first. Explain how much every one works or studies outside the house and see if they can understand that you can't do everything. I think people are often more willing to do chores if they are allowed to express a preference.For example in our house DS1 hates cleaning the bath but doesn't mind putting the rubbish out.
Could you pay for some help? It sounds as though you have a lot on your plate and if you could afford to pay even for 2 or 3 hours a week cleaning help it would really take the pressure off.
mrsjammi - have now read the other thread and there are lots of great ideas on there. You said you might consider a cleaner when you go back to work. Why wait? You are tired and stressed now.I don't know your financial situation but it seems to me this could make such a huge difference to your quality of life it would be worth it. Put the SSs lodge up and use it for some help. You might then find you are a happier, less resentful person and the general tension in the home would improve. You also want to enjoy that new baby when it comes.
Many years ago when I was 18 and working we added up all the household bills except for mortgage and I paid my "share" which was 1/3 as there were three adults in the house.
mrs jammi you have inspired me! After yet another Sunday night row over housework, state of bedrooms, ironing, puppy walking etc. I read this thread and have laid down the rules in our house too.
If they want to go out at the weekend they will have had to put out the rubbish, helped prepare dinner, helped wash up, walked the puppy, put their laundry in the laundry basket not on the floor, etc. etc. Not sure how long it will last this week, but when they don't go anywhere (and this includes the mall, friends etc.) the message might sink in.