/in/isSibling.c:1:1: error: unknown type name 'bool'; did you mean '_Bool'?
 bool isSibling(const char* phrase1, const char* phrase2)
 ^~~~
 _Bool
/in/isSibling.c: In function 'isSibling':
/in/isSibling.c:4:13: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  int len1 = strlen(phrase1);
             ^~~~~~
/in/isSibling.c:4:13: warning: incompatible implicit declaration of built-in function 'strlen'
/in/isSibling.c:4:13: note: include '<string.h>' or provide a declaration of 'strlen'
/in/isSibling.c:37:7: warning: implicit declaration of function 'strchr' [-Wimplicit-function-declaration]
   if (strchr(str2, str1[k]) == NULL)
       ^~~~~~
/in/isSibling.c:37:7: warning: incompatible implicit declaration of built-in function 'strchr'
/in/isSibling.c:37:7: note: include '<string.h>' or provide a declaration of 'strchr'
/in/isSibling.c:37:32: error: 'NULL' undeclared (first use in this function)
   if (strchr(str2, str1[k]) == NULL)
                                ^~~~
/in/isSibling.c:37:32: note: each undeclared identifier is reported only once for each function it appears in
/in/isSibling.c:46:10: error: 'true' undeclared (first use in this function)
   return true;
          ^~~~
/in/isSibling.c:50:10: error: 'false' undeclared (first use in this function)
   return false;
          ^~~~~
/in/isSibling.c:52:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^