it's the only way they can "help" you. so let them help. believe me when your further along, you will welcome it. it's also cute that they are being protective. it shows that they care and a lot of women don't have that. I was 20 weeks when my family found out I was pregnant, and was walking 9 miles a day, when my mom would come over, she wouldn't let me lift anything. Now I am 37 weeks, and I can't pick up food I drop on the floor! So it's a good time to get use to people helping you :)
My mom does that to me all the time. Enjoy it while it lasts. And you can always do stuff when they arent around. I have an almost 3 yr old and they dont say anything about me picking her up but heaven forbid i carry some groceries or something. Try not to see it as such a negative and remember they are just trying to think of the baby. Good luck.