Why is sin_addr inside the structure in_addr?

struct in_addr is sometimes very different than that, depending on what system you're on. On Windows for example:

typedef struct in_addr {  union {    struct {      u_char s_b1,s_b2,s_b3,s_b4;    } S_un_b;    struct {      u_short s_w1,s_w2;    } S_un_w;    u_long S_addr;  } S_un;} IN_ADDR, *PIN_ADDR, FAR *LPIN_ADDR;

The only requirement is that it contain a member s_addr.

Because the in_addr structure may contain more than one member.

http://pubs.opengroup.org/onlinepubs/009604599/basedefs/netinet/in.h.html

struct in_addr is a more than just an integer is because it might have more than in_addr_t. In many systems, it has a union, and the reason of such implementation is for class A/B/C addresses, which are not used now.

Unix Network Programming Volume 1 explains the historical reason in detail:

The reason the sin_addr member is a structure, and not just an in_addr_t, is historical. Earlier releases (4.2BSD) defined the in_addr structure as a union of various structures, to allow access to each of the 4 bytes and to both of the 16-bit values contained within the 32-bit IPv4 address. This was used with class A, B, and C addresses to fetch the appropriate bytes of the address. But with the advent of subnetting and then the disappearance of the various address classes with classless addressing, the need for the union disappeared. Most systems today have done away with the union and just define in_addr as a structure with a single in_addr_t member.