Arthur is 2 years older than Susan, so he is 15 + 2 = <<15+2=17>>17 years old.
Bob is 11 years old, so Tom is 11 - 3 = <<11-3=8>>8 years old.
So in total these four family members have 11 + 15 + 17 + 8 = <<11+15+17+8=51>>51 years.
#### 51