Is any of this shown in the game? No. All that is shown in the game is Tim had asked her to take him to that school fair. She had promised to do so. But lied and went off with Tom to the Mall. Besides if Tom and Lisa had been acting all that time like they were in the beginning of the game. Why would Tim want to go along and watch them playing kissy face like boyfriend and girlfriend instead of Mom and Son. And be the 3rd wheel?Her not taking him is the catalyst that kicks off the whole game.
For months and months...
Lisa = 'Wanna go with?'
Tim - 'No'
Lisa = 'Wanna go with?'
Tim - 'No'
Eventually, she stopped asking - and Tim was fine with that, until he realized that they had moved on without him.
Tim blames Tom, because Tom is now much closer to Lisa. Tom blames Tim because he sees it as Tim abandoning Lisa (and him) in her time of need (Lisa's had been all but abandoned by her husband, who is most likely cheating on her). Lisa finally realized how messed up things were between her and Tim when she assumed he would be ok with Tim staying home all day on their birthday. In her defense, why would she assume he would want to go with them when for months, he consistently said no to doing anything with them. When she tried to talk to Tim, he shut her down, and so she chose to ignore the problem instead of dealing with it.
But now I see who the favorite Son is. The favorite was picked before the game even began.