I'm still trying to put some of the pieces together as well. It looks like Charlie F. Bost was his son (he shows up in the 1880 census-and must have moved to TX with him). Do you have any dates for Charlie that we could comfirm this is the same person? I still have a couple of relatives to talk to, but currently I don't have additional information about him moving to Texas. I did read a post that John Harvey and almost all his brothers moved to Texas, although it looks like it was at different times. Because as you stated, Marcus appears to have been already in Texas in 1880 (per the census). I'll try to find this information again. John Harvey, wife and kids were still in NC in 1880.

"Household Record 1880 United States Census
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Household:

Name Relation Marital Status Gender Race Age Birthplace Occupation Father's Birthplace Mother's Birthplace
Harvey BOST Self M Male W 44 NC Farmer NC NC
Cyntha BOST Wife M Female W 46 NC Keeping House NC NC
Cora BOST Dau S Female W 12 NC NC NC
Charlie BOST Son S Male W 11 NC Works On Farm NC NC
Sue BOST Dau S Female W 8 NC NC NC
Ada BOST Dau S Female W 6 NC NC NC
Anna BOST Dau S Female W 4 NC NC NC
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Source Information:
Census Place Denver, Lincoln, North Carolina

Hopefully, we can figure out when John Harvey and his brothers moved to Texas and which kids went as well. I know John Harvey's daughter Anna stayed in NC (this is my line). She ended up marrying Harvey Calvin Troutman. His other children, I'm not sure about. But as you say if Marcus moved there first, then some of his brothers were likely following him. I'll keep you posted as I find more out, and any information you notice that I am missing on the website, please let me know.

Feel free to go to my website to for further detail on dates etc.

Thanks,
Jeff