Jowy and Teresa Look Alike
Please look at this picture. On the left side is Jowy Atreides, and to the right is Teresa Wisemail. The resemblance is uncanny. Not only do they both have blone hair and blue eyes, but they also have very similar, or nearly identical facial features. This should be enough proof to convince most people that Teresa and Jowy are siblings.

Jowy's Ring
When Jowy sells his ring at Muse, the teller says that the ring has a lot of worth. This is said to be because of the fact that the ring belongs to a family of high prestiege. However, if the ring was an Atreides signet ring, it would not make sense for it to have much worth in Jowston--the Atreides family is under the Highland Kingdom (And they are "third-rate" aristocrats). If the signet ring actually belongs to the Wisemail family, it would make complete sense why it would have been of worth in Muse.

Jowy's Dealing with Greenhill
Jowy volunteers to attack Greenhill when Luca asks for volunteers. Jowy risks his life by volunteering, and many believe he does this in order to realize his ambition. However, Jowy may also have had another reason--to ensure the fury of other generals conflagrate Greenhill like it did other cities. Case in point, many other towns or cities conquered by the Highland Army suffered heavy losses, such as Toto and Ryube which were burnt to the ground, Muse which had many of its citizens killed and it's mayor murdered, and South Window, which suffered the loss of their mayor.
Although Jowy must have had mixed feelings against the Wisemail family, he surely would not have wanted to have his sister to perish like the other mayors. That must be why he made sure he was in charge. He also elected to use a strategy that ensured no actual battle. He took meticulous care to not harm his sister and her city. This obvious "favoritism" can be seen from the fact that Jowy participated in the "pig hunt" organized by Luca to track down Muse citizens who escaped. Jowy wanted to make sure Greenhill was not touched by other Highland generals. He went so far as to appoint Yuber as the general in charge of Greenhill after he was forced to attack other cities because Yuber was one general who was directly under Jowy's reign. Why did he take so much care in handling Greenhill? Isn't it obvious?

Rosa Atreides
If Jowy was originally a Wisemail, why did Rosa leave the Wisemail family? Lets look at the timeline to see what could have possibly happened. Jowy is 17 years old at the time of Suikoden 2, which is the year 460. What happened in Greenhill around the year 443? It was around then when the Karaya Chief Kianu was murdered by Alec Wisemail and Gorudo of Matilda! Whatever happened, Rosa must have been disgusted with Alec. Perhaps Jowy was not yet born, or was too young to entrust with Alec. Teresa would have been 10 years old, and already integrated into Greenhill society, and that is probably why Rose didn't take Teresa with her. Who knows how Rosa came in contact with Marcel Atreides, however, it is understandable that Rosa Wisemail would not have been able to find a place within Jowstone to settle down--everyone must have known who she is, and Alec would have made sure she was smeared in every way, as she knew a secret no one else should have known.

Karaya Relations
Because the truth behind the murder of Chief Kianu was a secret that no one was supposed to know, it would be plausible to say that Jowy used this knowledge to recruit the help of Lucia. Rosa probably told Jowy the evil deed of Alec Wisemail, and he obviously exploited this fact in order to gain a new ally in his fight against the Allied Army. No one else, especially in the Highland Kingdom would have known of such hidden enmity between Greenhill and the Karaya Clan.

Teresa Wisemail
It is no secret that Jowy views Teresa with special regard. During the peace conference, Jowy demands that Riou and Teresa be present. His choice of Riou is obvious, as he is the leader. However, why did he ask for Teresa? If he is asking for Teresa, why would he also not ask for Makai? It is obvious why he asked for Teresa with Riou, as she is probably his sister. He must have thought he would be able to convince her more easily, as she is his sibling.

With all these evidences, very few would see any evidence to the contrary. Jowy may be of no relation to Teresa, but there really is nothing that says he can't be Teresa's brother. Canon or not, it actually seems quite possible that Jowy and Teresa could be related.