A portrait design is a tattoo of a photograph that appears life-like more than it does an artist's representation. For your brothers, consider tattooing a picture of both of them together or separate pictures on opposing parts of your body. For instance, consider a tattoo of the elder brother on your right shoulder and the younger brother on your left. Portraits have a high level of detail and generally need a relatively large and flat space to portray properly, such as your thighs, shoulders or back.
Script tattoos are one of the more straight forward styles and are ideal for someone that doesn't want a large, overwhelming design. Consider tattooing the first names of your brothers, again on opposing sides of your body in any font you wish. There are hundred of styles and fonts to choose from, but if you want to make the tattoo even more unique and personal, consider either designing the script yourself or having your brothers sign a piece of paper and tattooing their signatures.
A much less straightforward and more abstract way of memorializing your brothers is to tattoo a random object that represents each of them. Choose a significant item from either the past or present that bears some level of importance. For instance, if one of your brothers loves trains, consider tattooing a small train, or if one played with toy cars as a kid, tattoo one of those cars. Whatever the objects are, just make sure they have a shared importance between you and your brothers, even if no one else understands the meaning.
Ambigram tattoos are a style of script that does not necessarily work for all words. An ambigram design reads as two different words when you flip the image upside down. There are several ambigram generators online that allow you to plug each word into the system and will either generate an image of the two words or say it is unable to. If you absolutely cannot find an online source that will ambigram the names of your two brothers, sometimes the right artist will be able to do so on his own.