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...unless I'm missing something, there just isn't enough info here to give you a fair answer
...what state are you in
...what are the specific terms in the LC that apply to your final purchase
...do I understand correctly that you want to replace the LC with a regular mortgage? ...if so, read the link in my sig "Do I Qualify" and add as much of the needed info as you can here ...but if thats correct then I can't think of any reason another party can't co-sign if your brother agrees and the LC allows it ...hth