The gemara says that someone showed up claiming to be Mari's brother and demanding a share in the inheritance. Mari responded that he doesn't recognize him and Rav Chisda justified the claim, since when Mari last saw him he didn't have a beard and now he has a beard - and proves that it is a legitimate claim from yosef and his brothers. The gemara seems to imply that when a person shows up out of the blue claiming to be a brother, unless there is a valid explanation to explain why he doesn't recognize him, we would assume that he is actually a brother and give him a portion in the yerusha. Tosafos is troubled by this - דא"כ לא שבקת חיי לכל בריה העשירים שכל אחד יאמר אני אחיך. Tosafos argues that it can't be that someone can come and is beleived to be a brother (unless the other one has a legitimate reason why he wouldn't recognize him). Tosafos explains that had he claimed "he is not my brother" that would be sufficient. But the situation was that mari bar isak wasn't convinced that this person wasn't his brother and therefore did not make a definitive claim. Mari's claims is that "i don't know if he is my brother", meaning that he very well may be so it is a ta'ana of she'ma against a definite claim of "i am your brother". Therefore the other person would be believed that he is in fact his brother, unless mari can explain why he isn't sure such as that now he has a beard.
Rashi seems to deal with the same question much more simply. Rashi writes - שפיר קאמר לך שאינו מכירך ואין זה רמאות. Rashi (d.h. asa lei) explains that the case was that Mari admitted to having a brother overseas, therefore even though mari makes a definitive claim that he doesn't recognize this guy it seems like he is not being honest - therefore r' chisda justifies mari's claim that do to the beard he truly doesn't know whether this is his brother. The Rosh seems to go along the lines of Rashi. He explains the need for the legitimizing of why mari doesn't recognize him that the person is not just claiming to be a brother, but he is claiming that mari truly knows that he is his brother and is denying it. Therefore R' Chisda justifies why mari truly doesn't know.
The Nimukei Yosef seems to argue against all the other Rishonim. He understands that since it is completely uncommon for a person to show up claiming to be a brother, when the response is "i don't recognize you", it is "ניכרין דבריהם שזה טוען אמת וזה שקר שלא מצינו איש שיקרא למי שאינו אחיו אחי אתה, ומה שטוען זה הוא רחוק שלא ידע אם יש לו אח או לא". Therefore, in this type of situation where it seems like the person coming is in fact a brother, the burden of proof is on the other to explain why he doesn't recognize him. That is why R' Chisda had to justify that it is because he now has a beard.
Based on the Maharal (gur aryeh) it works out very nicely. The maharal explains that rashi is bothered why the pasuk repeats itself saying that yosef recognized them, twice. He explains that at first it is just telling the facts that yosef recognized them, and hid his identity so they don't recognize him. But then the pasuk says that it was possible for him to recognize them since he has seen them with beards, but it was not even possible for them to recognize him since they have never seen him with a beard. Based on this we can understand what R' Chisda is proving from yosef. Perhaps yosef just happened to recognize him and they just happened not to recognize him due to the beard, but not necessarily is it a great claim. However, based on the maharal R' Chisda was showing from yosef that he was confident that they wouldn't recognize him since it is impossible to recognize him with a beard.