One of my coworkers from a previous job fairly recently started dating a guy with 3 kids and she posts pictures of them all the time. At first I rolled my eyes at them like I usually do at stepfamily things. But taking a closer look - the kids look happy and like NICE kids. And even more - her boyfriends mom commented how thankful the kids are to have her in their life! I'd probably fall over dead from shock if my MIL ever commented something like that. In contrast, MIL passive aggressively excluded us (DS9 and me) from the beginning. The first picture ever posted of myself, my now DH and the kids she commented "MWAH Hi SD granny loves and misses you!" At the time I thought it was weird and kind of rude and since then I've come to realize that yes, she was trying to be intentionally rude because when BM was sending her pics of SD for awhile she told us she ignored BM by responding with "Hi SD, granny misses you!" So basically the same comment.
In addition, theres no way SD would ever want to take a picture with me, let alone be thankful for me. When we did do family pics once, SD was so pouty and told the photographer "that's not my mom!" with such a nasty attitude that the photographer just stared at her dumbfounded and didnt know what to say.
Seeing that makes it hit home that some of the things I've gotten used to arent normal or okay. Theres a reason some step situations seem to work, and in my jadedness I always assume its BS. But there are actually some nice kids and nice MILs.