OS/2 1.x was co-developed by IBM and Microsoft. There are some resemblances between OS/2 1.x and Windows 3.x APIs, because many of those APIs were actually designed by the same Microsoft employees – for example, most OS/2 API calls starting with Win* have a similarly named Windows 3.x API call, just without the Win* prefix. (Despite similarities like that, the APIs are incompatible; they probably would have been more compatible if it were not for IBM's influence – for example, OS/2 and Windows use different coordinate systems in their graphics API, because IBM insisted OS/2 had to use the same coordinate system as IBM's mainframe graphics software, GDDM.)

Microsoft and IBM have cross-licensing agreements allowing use of OS/2 code in Windows and vice versa. Microsoft used this to include OS/2 compatibility components in old versions of Windows NT (newer versions have removed it); likewise, IBM used it to include a copy of Windows 3.x in OS/2 to enable running Windows 3.x applications. (The agreement did not include newer software they developed after their breakup, so IBM wasn't allowed to use the Windows 95 or Windows NT code in OS/2, nor was Microsoft allowed to use OS/2 2.x and higher code in Windows.)

So, the odds of IBM trying to sue Microsoft over OS/2 APIs in Windows is zero. It would be precluded by the licensing agreement between them.