Property SIM2 demands that for all s1->s1' and s1<s2, we must find a s2' with s2->s2' and s1'<s2'.

The special cases that trouble you are therefore be dealt with as follows: if we have a state s1 which is in relation to a state s2, i.e., s1<s2, but there is no successor s1' of s1, then SIM2 is trivially satisfied regardless whether s2 has successor states or not.

If s1 has a successor state s1', but s2 does not have a successor, then SIM2 is not satisfied and therefore we have to remove (s1,s2) from the current relation.