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By Scott Stump

A day before a memorial service is set to honor the MIT police officer killed in the aftermath of the Boston Marathon bombing, his siblings remembered a kind and charitable brother who was dedicated to his work.

Sean Collier, 26, was found shot to death in his vehicle on the MIT campus on April 18, allegedly at the hands of Boston Marathon bombing suspects Dzhokhar and Tamerlan Tsarnaev. Vice President Joe Biden is expected to travel to Cambridge, Mass., for Collier’s memorial service on Wednesday. In an interview that aired TODAY Tuesday, Savannah Guthrie spoke with Collier’s sisters, Jennifer Lemmerman and Nicole Lynch, brother Andrew Collier, and step-siblings Jennifer and Robert Rogers.

“It was a nightmare come true,’’ Lynch told Guthrie. “I think everybody is just going through the motions right now and pulling together for Sean because that’s what he would want us to do.”

“I think we are coping as best as we possibly can,’’ Lemmerman said. “We are a very large and close family. We've sort of rallied together at our parents’ house, just spending time together and trying to remember the best parts about Sean.”

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Sean Collier was killed on April 18 while on duty on the campus of MIT. Today

Collier's siblings said his concern for others was apparent at a young age. Andrew remembered his brother scooping up insects in the house and taking them outside instead of killing them as a kid, as well as his desire to help others.

“Sean was such a good person,’’ Andrew said. “(I remember) stories of being at restaurants and seeing a woman crying and all of us are sitting there kind of just doing our thing eating our Papa Gino's, and Sean just obsessing that my mom needs to go over there and try to help.

Video: Remembering fallen MIT officer Sean Collier

“That really shows from the very start, Sean's just care for other people, and it’s almost something that I took for granted and didn’t realize, but now through his death, I realized how good of a person he was, and I wish I could have told him that while he was still here.”

They also described his instinct to protect, foreshadowing a future career in law enforcement.

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“Ever since we were kids, Sean and Andy would be doing something to each other, and then Andy would take off, Sean would chase him, and he would be doing siren noises and saying, ‘Pull over, you are breaking the law, you are breaking the law,’’’ Collier's stepbrother Robert Rogers said. “So ever since a very young age this is what he's wanted to do, and he did it. He worked his tail off, and he really has made us all very proud of him.”

MIT Police Chief John DiFava described Collier as a “dedicated officer who was extremely well-liked by his colleagues and the MIT community,” and the family has received an outpouring of support and gratitude from those he helped along the way.

Collier, right, with his brother. Today

“That’s been something that's been really just amazing for our whole family because we see a side of Seanny that he's my little brother, my jerky little brother who is too cool for me,’’ Collier's stepsister Jennifer Rogers said. “We have been able to hear about him volunteering, taking time out to personally meet students at MIT that he worked with, playing sports in the community with his friends. We get to see a whole other side to him that he was just so humble and didn’t really speak to us about.”

The family has asked that donations in Collier’s name be sent to the Jimmy Fund, a Boston-based charity that raises money for the fight against cancer. Back in high school, Lemmerman and Collier were at a summer job together at a medical office when Collier heard about a Jimmy Fund fundraiser on the radio.

“They just told stories of these children that were fighting for their lives and their families, and Sean was just so impacted by that,’’ Lemmerman said. “He went home and made a donation. He's just this high school kid. He had no money, but he made a small donation.

“Then he set up a recurring payment from his own account to make regular donations to Jimmy Fund, and he did that until the day he died, so we wanted to carry that on for him.”