Glad to hear you taking action OP. I too fail to see how anyone, no matter how nice a person they may appear to be, can be oblivious to such exclusion.
Myself and my 2 younger siblings dealt with a lifetime of this obvious ostracising from SFs family. 3 of us made to sit together silently in a corner while SF, half SIS and my M got showered with gifts and had cheerful happy family photos together while we just watched on. The gifts weren't the issue, it was the blatant "you are not family, merely your mothers tagalong's, make yourselves invisible whilst with us" that made me choke back tears at every 'family' occasion. I'm still angry at how my mother allowed it.
And these people were devout Christians,
Years later I still tried to include them and invited them to my wedding. They didn't come, didn't even have the decency to RSVP and decline, simply told SF to tell me "it's too far to travel". However(!) a week later after my wedding I got an email from one of them asking for the 'lovely photos' from my wedding.... but only the photo's containing SF, M, half sis and SB and his family. Not, me. Not my siblings. Not my DS or my DH. No, why would they want them!
That hurt was felt all over again, even as a grown adult.
You are just leftovers from a past we don't give a shit about is what it says when you start excluding step children with something as simple as a token gift.