Yes.
To form a triangle every side must be smaller in length than the sum of the two other sides.
For example: sides denoted by a,b,c
for a triangle to form
a<b+c
b<a+c
c<a+b
Thus in the case of the
Isosceles triangle with side lengths 1,8,8 the rules aforementioned are fulfilled. This means that a triangle with said side lengths can exist.