So many problems with this.
If they can't afford the child, they shouldn't get a child.
Your "I don't consent to be the mother of the kid" is so so very naive. IF you are going to go through with this, you WILL need to have an independently hired lawyer (that you guys hire!) paid for by your bil to draft a very long formal contract so you won't end up as a parent. Mind you, these contracts are 20+ pages long and very complicated.
Why this is necessary - there's been many cases where the "biological mother / father" a been sued for very expensive child support after the deed. They can say, "I only agreed to donate sperm, etc...", courts only care about what's best for the child. It doesn't matter if you had mutual agreement. You definitely don't want this to happen to you.
Again, you WILL have to go through agencies to have your ass covered. If you really want to cut costs for the sake of saving them money, you're putting yourself at high risk if the couple can't take care / if they separate.
Don't do this. If you really want to, don't cut corners for them, proper lawyer, proper agencies, proper contract. Don't set yourself on fire so they can save some money.