My mom has Guardianship of my sister who is 27... she is mentally ill. She's not very bad with it... but is not able to live completely on her own. She is living with my mom. My dad and mom have been divorced for quite a while now. My mom, sisters and I live in Iowa and my dad lives in our home state South Dakota and my dad was wanting her to come up and visit for a month since my grandma is getting bad. She has Alzheimer's but she still knows who we are but sometimes gets confused. My mom will not let my sister go spend a month with them. Since my mom has guardianship of my sister does that mean my dad could too? Just as if she was underage.. when he had joint custody of us?

Here is what you face. By what you have said she (your sister) is of reduced mental capacity but not unable to make many decisions on her own. In the courts eyes it would be unreasonable for your mom to not allow a visitation so long as all the dots are connected. Meaning she is cared for the whole way. If she has the mind of a child and the body of a woman you wouldn't want her alone on a plane but should someone accompany her then it might be reasonable.

The courts wouldn't look to have both parents as gaurdians as if she were a child. They prefer at this point to have a single guardian or couple and not 2 persons in separate homes or states.

In the case of a minor child that child can't do for their own self and that is why its both parents responsibility. But with an adult child then only a single parent is needed. Minors can't sign for anything. Your sister may be able to sign for some things. What is the nature of her diminished capacity? If its skirting the bounds for living alone but still aware of the world and has a partial education then they can't be held against their will.

They are free to visit whom they like so long as its in a safe environment.

State laws vary, and so do terms of guardianships, but a parent doesn't lose his rights because another parent is guardian. Your father can ask the court (probably in Iowa) to spell out what your sister is free to do or not, and what visits you can ask for. Courts want adults who have fairly mild inability to live on their own to have as many rights to decision making as possible. Maybe he can get a copy of the original papers and start from there. If your sister just can't handle finances or cleaning or tends to recluse, she should be allowed to go along with you. Try to talk this out. Maybe your mother just feels that she can't handle a whole month, and you could shorten the visit.