V.,
It sounds like you are in a tough situation, and being pregnant on top of it probably isn't helping your emotions. It also sounds like you have a loving and caring MIL who is just trying to make the best of the situation. She probably isn't interested in invading your space as much as just trying to be helpful and do something that she sees that needs to be done. It is her home, and she is used to doing the work to maintain it. You need to respect that. Try to look at it from her perspective and imagine that your children are grown and gone and then one day one of them needs to come and live with you, bringing their spouse and four children along.
You may try talking to her and letting her know how much you appreciate all they are doing for you and that you feel in return it is only fair that you do some of the work around the house. You can start by saying that you feel it is only fair to be responsible for the rooms you and your children are staying in, and you will do the work associated with your living space. Sheets, dusting, tidying, etc. and so on. In addition, with the extra people around the house I'm sure there are extra chores (expecially with kids) as a result and you could offer to help her around the house in general (vacuuming, dusting, etc). Perhaps you are already doing those things, but if not it is something to consider.
Because you are living with grandma, she probably feels a certain amount of responsability for the comfort of her grandchildren. After all, you may be living with her but in a way you are also guests and she probably wants her guests to be comfortable. So, while you may find her suggestions offputting to you she sounds like she is just wanting to help her grandchildren. Don't be harsh with her or get upset, but say politly that you appreciate her suggestions however this is how you and your husband (or however you want to phrase it) have decided betime should be and you want to stick with it until you determine a change needs to be made. A child crying for a couple of minutes is not so bad, and they have the "company" of the other kids in the room with them so they are not alone and they know that.
Things will get better as you move on to your own home again. Just know this isn't a permanent thing, but it could last for a while longer. Discuss your concerns with your husband, and look for opportunities to get out and do some things for yourself. You and your husband should also get out together once in while. After all, living with the grandparents in a way means live-in babysitting! Take advantage while you can of the opportunity.
In addition, you should also be considerate of his folks and try to provide them with some private time as well. Again, remember that while they may be welcoming and glad to have you that you are guests in their home and the saying that "it is nice to have company but that it is also nice to see them go" is something most people do relate to. Maybe you are already doing this, since there was at least one opportunity for your MIL to change your sheets while you were gone.
You are needing to make the best of a tough situation, and I'm guessing that you want to feel more in control of things than you do now. Sometimes it is nice to not be in total control, but it can be hard to let go enough to enjoy it. Try to enjoy this time if you can, and appreciate the help you are getting while you are pregnant. With four other children already you probably get tired out pretty quickly I can imagine.
I wish you all the best.
J.