My sister is pregnant. Medicaid is covering the delivery, but my sister wants me to adopt the baby. Would I need to cover the cost of the delivery?

Do you actually want the baby? or are you just doing it as a favor to your sister and to keep the child in the family. Are the boyfriend's parents involved in this? They may want to raise the baby themselves.
You definitely need professional advice with this to protect your rights and the rights of the child in the future.
I personally feel that adoption should be done as soon as the baby is born and completely final so the new parents have absolutely no interferrence with their bonding with the child and no interference with the upbringing.
You have to be prepared also for the birth parents to change their minds once they see the baby.
I don't think there should be any costs associated with the prenatal care or the birth. The boyfriend may be required to reimburse medicaid but once adopted he should not be required to pay child support. your State could have rules for adoptive parents so all that needs to be checked out.
It is a wonderful thing that you are doing for your sister but it must be clear she has given you all rights and she can't just breeze in and play Mommy anytime the fancy takes her. I

I know this is not relevent but a neighbor asked me to adopt her dog as she and her partner would only be home at week ends. I agreed and took care of the dog but every week end when they arrived the dog went to their house and did not return till Sunday night when they left know this is hard nosed but you two will be the parents not child carers.

I second the idea of contacting your employer, to ensure that an adopted baby could be covered. I suspect there are some specific requirements though.

Are you considering a private adoption? If so, I'd recommend considering an adoption through a government agency to ensure everything is done properly.

I worked in the Juvenile Division of Probate Court back in the mid 1960s and have some recollection of having an adoption specialist on staff to handle adoptions, but don't recall for sure. You could contact your local county Probate Court and see if it handles adoptions. There are probably costs associated with it, but it would be done properly and in accordance with government standards.

The last thing you would want is to find out that the adoption wasn't done pursuant to statute or perfected properly and there were problems on down the line.

Be prepared for a background check by any governmentally involved agency.

I can think of one specific issue that will need to be addressed, and that's the rights of the parents to visit, take the child for outings, etc., and whether or not there would be any compensation to you for this.

Well hopefully you are using an attorney. The trick will be to find an experienced attorney in this area. My opinion is that no, you are not your sister. She is getting health care for the delivery of the baby. When does the babies care become your responsibility for health care is I suppose the question.
Perhaps someone will have actual experience with this but it’s too important to take a chance on.
Call your insurance provider to see what the rules are for the addition of an adopted child. I would imagine they have a firm policy.
While this is an important question, I would be having many more.
Congratulations to your family. Hopefully it will work out the way you envision.