As others have said, if you don't present the poster, you can't put it on your CV. See if there is any possible way to have someone else present it for you. If there are other RAs in your lab, ask them about this before you approach your PI, so that you have a potential solution in place before you explain that you can't attend. If no one else can cover it, consider whether there is any way that you can make it to the conference. It's a 7 hour drive from Baltimore to Boston, so if you hit the road that morning for an afternoon poster session, you could be there in time to present, and then you can head home again. It won't be a pleasant day, but still better than bailing on the poster. If it's a morning poster session, drive up there the night before and then head out after your poster session, which will require 1 night in a hotel instead of multiple nights. It's not ideal, but at least you can still present and put the poster on your CV.
While some may feel that the feedback in this thread is harsh, you are likely to encounter people in the field who share the same opinion that it's poor form to skip a conference when you were scheduled to present. An undergrad RA in our lab had a poster accepted and then decided not to attend a conference once, and our mentor was really angry - not just because she wasn't honoring her commitment, but because others had also put time into helping her with the poster. I was able to take over the poster, so it got presented, but neither of us (mentor or myself) would have put time and energy into helping the RA with the poster submission in the first place if we had known that she wasn't going to present it. I don't need any more posters on my CV at this point.