32 weeks pregnant and having some at home issues,need advice please?

im 32 weeks pregnant with my 3rd child. as of now i stay at home with my kids while my hubby works full time. my sister in law and her 2 month old moved in with us as she needed somewhere to stay for now. i love her and care about her baby but im stressing big time. shes on maternity leave until later this month. i have been cooking everyday for everyone including her,watching her baby when she runs arrons,clean after everyone including her and its causing me to feel achy and stressed. i havent been able to enjoy 1 month of my pregnancy,and for once at least the last weeks i have left i would like to relax. when she goes back to work im going to have to watch her baby,my 2 yr old,my new baby coming and my 6 yr old and prob cook and clean as usual. she sees me cleaning and cooking but doesnt offer to help etc. when she was prego i helped her through everything. i do not want to say anything to her but wanna know what i should do??? im done! please alla dvce is great thx......