MS-DOS (from the x86 DOS lineage) technically starts this lineage, as Windows was simply a graphical shell for it at first. By the time of Windows 95, Windows was a full-fledged OS but still had DOS underpinnings and relied on DOS as a bootloader, and the lineage ended in 2000 with the debut of the last 9x, ME.
MS-DOS-tan and PC-DOS-tan were cloned from QDOS-tan, and identified as being from the x86 DOS (then CP/M) bloodline, but were named as the start of a new lineage, along with their sister Windows 1.0-tan in 1985. Their upbringing, culture, and the ties that MS-DOS and PC-DOS had to the early Windows-tans marked them apart considerably from the rest of the DOSes.
Originally, MS-DOS and PC-DOS saw the other DOSes as competition and didn't think much of them until many years later.
The Win 9x moniker for the DOS-based Windows-tans comes from the prominence of 95-tan; her predecessors mostly lived in obscurity, 1.0-tan and 2.0-tan aren't even acknowledged within the Windows Family.
Traits and abilities
The 9x's have widely varying hair colors and eye colors, but all of them are fairly petite (the tallest of them being 95 or 95 OSR 2.1). Their abilities have a lot of variation, such as 95's sword-wielding, 98 and 98 SE's mech piloting and ME's scallion-wielding, but all of them have the ability to harness the DOS command line and programs.
All of the DOS/Win9x-based Windows-tans were born into the Windows Family or its predecessor (except for Windows 3.2-tan and 97-tan), but not all live there anymore, as some of them were exiled (1.0, 2.0), or defected (95 OSR 2.1, 2.5).
The NT branch and part of the 9x branch (composing of the canon Windows-tans plus MS-DOS) are united in the current-day Windows Family. They don't know the two branches aren't blood-related, let alone the DEC/VMS heritage of NT, as a lot of secrets have been kept from the younger generation Windows-tans, but happily treat each other as sisters nonetheless and are a close-knit family.