Didn't they say..
They had known each other (the Mother's) since the son was 5 years old and the daugther was 3 years old? According to this review it says the son didn't know the daugther that long.
>>>She strongly opposes his engagement to Kelly Shaw, who he doesn't know very long,<<<
Overdomineering female police detective Jamie McDowell still tries to run her 19-year old only son Peter's life although he is a builder who lives alone.
[spoiler]She strongly opposes his engagement to Kelly Shaw, who he doesn't know very long,<<< and forces him to bring her to diner, where Jamie and mother Susan Shaw repeat that mistake.] After Kelly provoked her ex-fiancé Ray Rankin in a bar 'to test Peter's love' again, they part in anger. That evening the mothers, who went out together, find Kelly burnt to death in Susan's house; an expert clearly concludes arson. To her horror, Jamie quickly finds the evidence seems to point at one logical suspect: her son Peter...[spoiler]