I don't think Margaret Trelawney was mother of the immigrant John Martin. However, I think it is very possible that she was his step-mother. I found a source showing that Margaret Trelawney was married in 1633, Plymouth, Devonshire, to John Martyn, and that would be after John Martin's wife (Suzan Hemertin) died in 1623. I guess that would make John Martin the immigrant (b. 1618-1619), a step nephew of Robert Trelawney (related by marriage, but not a blood relative).
I assume she was the Margaret Trelawney born in 1602, and her brother Robert Trelawney was the one born in 1598 (both born in Plymouth and both had father named Robert Trelawney).
The 1633 marriage might have been a second marriage for Margaret as well (not sure). Anyway, here is the source I found for the 1633 marriage record of Margaret Trelawney and John Martyn:
"England Marriages, 1538–1973 ," database, FamilySearch(https://familysearch.org/ark:/61903/1:1:N2SB-8Z9 : 10 December 2014), John Martyn and Margaret Mrs Trelawney, 28 Nov 1633; citing Saint Andrew,Plymouth,Devon,England, reference , index based upon data collected by the Genealogical Society of Utah, Salt Lake City; FHL microfilm 481,081, 823,684.