Abby I'm so sorry for the loss of your sister. Grief is hard work and you are way too early in this process to make such huge life decisions, it's like mixing apples and oranges. I'm glad you feel that he is supportive now..you need all the support you can get but I don't think is should have anything to do with "relationships" at this point in time.
I understand that honesty is important to you...I have to wonder if the "neighbor" wasn't offering the support you so desperately needed.
I'm glad you have found this forum...talking about our feelings can be very healing.