Finally, I realized why the proposed motto did not "sound right" with me: there are two words for "the other" in latin: alius and alter. The first one has a connotation of difference, or segregation. It led to the English "alien" with all its consequences.. The second, on the other hand, lies at the root of "altruism" - and this is probably what you would like to capture in your motto. I thus propose: "Pro alteris in periculo" (the modified order also avoids the misunderstanding that the helper might be the one in danger..)