A large print, bold comic sans troll.
Im not a step parent either, but im going to throw another "smokescreen" into the mix. I would save whoever cannot help themselves.
If, say, my nephew (by law, not biological) Jarrah, age 3, and my daughter Dita, age 17, were for example, caught in a rip tide, and I was equal distance to both, I would save Jarrah without a second thought, because he is 3 and cannot help himself.
If my son Sasha (4) and Jarrah (3) were in the same situation, the best I could do would be to assess who is in the most trouble and who needs my help first.
Of course, this is hypothetical, and chances are, if I were in a rip tide, id be the one needing saving, moreso than my 4yr old, who can swim better than me, but the point is still there. You are also very rarely in a situation where you have equal chance to help 2 or more people, there usually are risk factors and differences between the two.