My MIL wants me to sign a paper stating that if my husband and I divorce I give up custody and no support. Is that legal?

Where are you? This is an International site.

In general support is Court ordered along specific guidelines. Of course, if you lost custody you wouldn't be entitled to child support. Or are you asking about spousal support?

Where is your husband in this arrangement? He is allowing this to happen, for this "deal" to be made to you?

Why would you consider signing such an agreement whether it's legal? If you are unhappy in your marriage, fix it now.

It might be legal if you sign it under your own free will. But she can't force you to sign.

She us loaning us money. In return she wants me to sign a paper stating that if we divorce I walk away. No child support and my husband gets custody. She says it is to ensure we stay together and also so that if I were to leave he is still able to pay back the loan.

It sounds like she wants you to sign this as a prenuptial agreement? If you are in the US, no, you cannot sign away your rights nor your responsibility for your children. Nor can you relinquish your children's right to be supported by the other parent. Child support is the right of the child, as well as access to both parents.

In fact, if this is intended as part of a prenuptial agreement, you cannot address the disposition of future children at all in the agreement. These matters will be determined if and when you divorce.

The only way you can relinquish custody of a child in the US is if you place that child for adoption, or if your rights are terminated by the courts for cause. The cause normally has to be very serious, such as substantiated abuse or neglect of the children.

Why is the mother in law involved in your decisions for your children anyway? My response would be to tell her to mind her damned business.

OK, I just read your last post - about it being a condition of lending money. No, this isn't legal in the US. I would not accept any money that came with such intrusive conditions.

Pretend she is too poor to give you any money - you'd still find a way to survive. Do what you would do if this money didn't exist - it's toxic.

Your children's right to have two parents and be supported by both is not for sale, ever. This type of agreement would not be enforceable in court, but if you were to sign such a thing it could be used to show that you were willing to sell your rights to your children, which would show poor character on your part, so absolutely do NOT sign it.

If she is loaning you money tell her you are willing to sign a promissory note agreeing to pay back the money. But she has no right to dictate your relationship to your children.

This is not legal in US.