First and most important: Line up your arrangements and help for your kids now, where you are. Where's your husband in all this? Do you have other relatives or friends nearby to help? Can your two older kids help with the younger ones at least some? Because this trip to SIL should not happen, and if you have plans lined up, your SIL (and husband--?) can't press you to send them.
Please follow your instincts here. The fact you're posting about it tells me you're questioning yourself but please -- don't! What you describe would instantly mean no to me; even if the one and only issue were "she leaves them with a babysitter several times a week so she can go clubbing" it would be reason enough not to send them (you don't know the babysitter, and if she's clubbing several times a week, she's likely tired or even hung over several mornings a week--not a way to be supervising young children).
If she is lax with two, she is not going to suddenly shape up and be attentive with a total of six kids there. She may just let the oldest two do what they like ( do you really want that?) and will treat your younger two just like she does her own.
I find it very interesting, though, and worrying, that you don't mention even once what your husband says about her offer. At all. She's his sister. Why have you, and not he, been the one telling her no so far? Is he pushing you to say yes? If so, you have to be very firm and stick to your guns here. Lay out for him what you laid out for us here. Tell him this is not negotiable. We're not talking about a visit with her for a few days with one of you parents present; we're talking about your children being 3,000 miles away from you for as long as a MONTH.
Your oldest child very likely would end up being expected by SIL to babysit the younger kids and her own kids too, I would bet. Is your oldest ready for babysitting duties for five kids at once for a month?! That will give you one angry and resentful teenager when this is over. Of course your oldest can indeed help you out -- back at home, not a continent away at SIL's house. If your kids are at home, others can give your oldest two a break in helping you but at SIL's they both are going to end up bored, possibly in trouble, and babysitting.
You also don't say if there is a boyfriend in SIL's life. Is there a man you don't know very well hanging around her? If she's out clubbing is she also bringing people home? Who are her friends and who babysits? I would not have my kids in a household where I wasn't certain about who was coming or going, or if there was a significant other there whom I did not know extremely well.
So there are the reasons. Now: Why are you so worried about her feelings? Your post indicates you want to tell her no "gently" but your previous two "no" answers have not sunk in. Tell her: "The kids will not be coming to you if II go on bed rest. We have made other arrangements here for all of them. It's nice of you to offer, but we are not sending them anywhere." If she keeps asking about this, you may have to have your husband tell her this -- she may just not be listening to you, period.