Letter of Rec from Lab PI/Brother I am a college student considering applying to PhD programs both in the US and internationally. Currently I research in a lab affiliated with a top US university and run by my older brother, an instructor at that university. I really enjoy my research here and hope to continue with it; however, I worry about getting letters of recommendation for grad school. I will ask professors in project based classes at my university, but if I continue with this lab as I hope to, only my brother will be qualified to write one about my research abilities. Although the group has other affiliated instructors, my brother oversees the division I research in and my work. Will an admissions committee simply chalk a letter from him up to nepotism or, provided I demonstrate meaningful research, understand the circumstances?
I am of the opinion that as long as you disclose that he is your brother, you can still get a LOR from him. This is one reason why most universities generally want at least three or four letters of recommendation. (Maybe not specific for the sibling case here, but more broadly just the various nepotism cases). 

Assuming that your brother would be qualified to write you a LOR if he did not happen to be your brother, I personally would not be too worried about having him write you a letter of recommendation. He does need to focus on the facts and not speculate on future potential or future outcomes too much. If he states the actual outcomes you have accomplished in the lab, him being your sibling becomes somewhat less important. 

I would have something at the top of the letter and in your statement of purpose that very briefly discloses your relationship:


  Professor Jadir Grupta is my biological sibling [unless you are adopted I guess]. I have asked him to write a letter of recommendation based solely on the merits of my work in his lab.


Your brother could write something similar:


  @taurus is my biological sibling. I am providing him a LOR based on his academic work in my lab, not on the basis of our personal relationship. 


Every LOR I have ever written is for someone that I am biased towards liking. I could even recommend some of them more highly than my own sibling or spouse or child. At the end of the day, most every LOR is coming from someone who could show nepotism towards the candidate.