I am a 20-year old, single female college student and share a duplex with 3 other female roommates. We all signed the lease. My roommates have decided they will not at the apartment over the summer and one asked if her brother could live in her room during the first summer session. I told her that I am not comfortable with that request as I will be there by myself and I only know him from meeting him a few times when he visited his sister. Though our lease allow for subletting with the landlord's permission, my understanding is that the landlord would also need all of the current tenants to agree to any sublet arrangement as we all signed the lease. Hence, we are legally joined by the lease as "tenant" of the dwelling. I believe as our parents guaranteed the payment of the lease, they also would need to provide approval. Is my understanding correct?
You should read over your lease document, which may describe specific rules for subletting the place.
If it does not, then the basic idea is that you do not need to let in any subtenant or replacement tenant that you don't want to. You might consider writing an email or letter (and keeping a copy!) that simply says you don't consent to admitting this guy as a subtenant or cotenant. You don't need to say why. This can be sent to your cotenants or your landlord or both--they are all obligated to respect your rights.
Unless there is specific language giving the lease guarantors (like your parents) the right to refuse a sublet or lease modification, they generally do not have that right. If the nature of the rental changes enough that they are being made to guarantee a situation substantially different from what they signed up for, their guarantee might not be enforceable. But it is you, not the guarantors, who have rights as tenants.
I get the sense from your post that the potential subtenant's gender is one cause for your concern. Your objection to his tenancy on those grounds may raise your roommates' hackles, but in Fair Housing Council v. Roommates.com, the court affirmed that roommates can discriminate against each other for whatever reason they want, including gender, even though landlords generally can't. So you're in the clear on that point.