Technically it really shouldn't if all goes well. However in the future if you and your boyfriend are unable to continue paying during the term of the lease agreement your brother may be stuck with the rent of his place and yours. Cosigning is basically an agreement that if you and your boyfriend are unable to pay, your brother will .
I usually recommend against cosigning but it is ultimately a personal decision.
Best of luck to you, your boyfriend, and your brother.
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I generally tell co-signers NEVER to do it, as it is essentially agreeing to pay for someone elses stuff without the benefit of actually having it! While true, if nothing goes wrong, everthing will be alright, this view is the very lifeblood of why we have lawyers, lawsuits and courts....because things sometimes simply go wrong. As a lawyer I always counsel people to plan for the worst and hope for the best.
I think you should make different plans as it is obvious that you may be getting in over your head. You are short $900.00 walking in the door, and won't start work for almost 4 months. This is a recipe for disaster, as there are a myriad of things which can go wrong. You lose the job, get hurt, break up with BF, fired during probabtion period, etc etc etc, all of which have your brother on the hook for your rent or watch his credit get trashed and possibly sued. If your rent is $2,400.00 a month as I think you are stating you would be wise to find an apartment that is more affordable for the short term, maybe even a 6 month lease, and then move to a fancier place when you have an established source of income you can rely on.