Justin failed to understand Monica in any way. Yes, her sister's death brought sadness to the family. Everyone we're devastated when Alisa died but Monica had to understand
No matter how much they try to live in the past, there was always a future waiting for them to continue living but she thought Victor opening up to something new was him forgetting about Alisa
"Sweetie, try to understand...I know and feel your pain but...Vic needs someone to get him out of depression" Justin murmured as he moved close to Monica and cupped the sides of her face. She cried uncontrollably and he hated seeing her in tears. Alisa was her only sister and the only family she had when she died so unexpectedly. Monica had it worse hence he understood how she felt