Step..?

time sensitive question.. hoping for a quick answer. Is a birth parent's spouse a step parent? background - my MIL was a BP, reunited with her son, M, when he is an adult. My MIL died in 2000. M as listed as my MILs son in HER obit. M talked to my FIL every month. My FIL died on Sunday. We are putting together his obit. MY SIL initially left him out of the obit (though she listed his other step sons). We got him included, but S added him at the end, telling my DH "M is not a step son" I imagined someone relegating my DD to the "other" section of an obit and got all "mother bear" I argued he was and got him added with the other step sons. overnight, i started wondering if am wrong. hoping someone on a.com would have an opinion.