Is this the Mindwarp (1997-1999) series by Chris Archer...?
This summary from TV Tropes outlines the general premise, mentioning five kids who discover that they have powers and silver blood when they turn thirteen, and a shapeshifting alien who shows up to capture or kill them:
In the sleepy town of Metier, Wisconsin (French pronunciation: MEET-e-AY), strange things are happening. Last year, Todd Aldridge disappeared when wandering near the reservoir (formed by a meteor impact) the night of his thirteenth birthday. Some people swear they saw strange lights in the sky. Now, in rapid succession, five kids (each missing one parent who all vanished on the same night) turn thirteen, discover superpowers, their blood turns a silvery color and a shapeshifting alien shows up to capture or kill them. As the series progresses, the kids organize and fight back. They even find Todd.
Book 1, Alien Terror (1997), features a boy named Ethan Rogers who discovers that he's half alien and a master of martial arts and weaponry:
Discovering that he is half-alien and a master of the martial arts and weaponry, former wimp Ethan Rogers finds his happiness short-lived when he is forced to fight for his life.
Book 2, Alien Blood (1997), features a girl named Ashley Rose who learns that she's also half alien and capable of staying underwater indefinitely:
Falling through the ice and facing certain death, Ashley Rose suddenly learns that she is a half-alien and capable of staying underwater indefinitely, but she also finds that she and others like her have been targeted for termination.
Book 3, Alien Scream (1997), features a boy named Jack Raynes who suddenly develops the ability to speak several languages:
Ignoring the anonymous note he receives on his thirteenth birthday that warns him his life is in danger, Jack Raynes is astonished when he suddenly develops the ability to speak several languages and overhears a plot for his own murder.
There are ten books in the series altogether and some of the later books apparently involve time travel to a distant future version of the kids' hometown of Wisconsin.