It's a bit of a stretch to connect Rebecca and Maurice on the strength of just a father's name in common and proximity in the 1881 census, but I think later events (starting with their death registrations) will support the assumption. (And maybe siblings of Charles and Eliza.)
Patrick and Rebecca had at least the following children (I'm guessing) —
Rebecca born about 1851 in Ireland, married James Connelly in 1876 in Ireland, last sighting 1901. Maurice born about 1852 in Ireland, married Rose Kammat in 1875 in Greenock, last sighting 1881. Charles born about YEAR in Ireland, last sighting 1875. Eliza born about YEAR in Ireland, last sighting 1875. Charles and Eliza qualify on the strength of their surname and status as witnesses to Maurice's marriage. It is always possible that they were a married couple, or cousins, or total strangers picked up off the street.
Aye, it'll be fun chasing up this lot.
1875 On Monday, June 7, Maurice married Rose Kammat at St. Mary's, Greenock. [See below for further information.]
1876 On Thursday, January 13, Rebecca married James Connolly at St. Mary's, Convoy, Co. Donegal. [See Section CN2-6-1 for further information.]
Maurice Slavin was born about 1852 in Ireland to Rebecca O'Donnell ( - ) and Patrick Slavin, Farmer ( - before 1875).
Rose Kammat was born about 1854 in Ireland to Mary McGovern ( - ) and James Kammat, Farmer ( - before 1875).
1875 On Monday, June 7, Maurice Slevin and Rose Kammat were married at St. Mary's, Greenock after Banns according to the Forms of the Roman Catholic Church. The marriage was registered No.156 at Greenock [Old or West] on June 9.
Maurice was a Bachelor, gave his age as 23, occupation Quarryman and usual residence 27 Charles Street, Greenock. His parents are recorded as Patrick Slevin, Farmer (deceased) and Rebecca Slevin m.s. O'Donnell.
Rose was a Spinster, gave her age as 21, occupation Farm Servant and usual residence 75 Holmscroft Street, Greenock. Her parents are recorded as James Kammat, Farmer (deceased) and Mary Kammat m.s. McGovern.
The celebrant was Henry Morgan and the witnesses were Charles Slevin and Eliza Slevin.
Maurice and Charles signed, Rose and Eliza made their marks.
The marriage is noted in the Register of Marriages in St. Mary's, Greenock. [Index entry]
Maurice and Rose had the following children (that I know of) —
Patrick born in 1877 in Bellside, Cleland, last sighting 1881. Susan born in 1881 in Bellside, Cleland, last sighting 1881.
1877 Patrick Slavin was born at 3h0m p.m. on Saturday, May 5 at Bellside, Parish of Shotts to Rose Slavin m.s. Kemitt [sic] and Morris Slavin, Stone-quarrier who married on June 7, 1875 at Greenock. His birth was registered No.46 at Cleland [Shotts West] on May 22 by Rose Slavin, "her X mark", Mother.
1881 Susan Slavin was born at 10h0m p.m. on Sunday, February 6 at Bellside, Shotts Parish to Rose Slavin m.s. Kammat and Morris Slavin, Quarryman who married on June 7, 1875 at West Parish, Greenock. Her birth was registered No.20 at Cleland on February 26 by Maurice Slavin, Father.
1881 April 3 The family were living at 3 Carlisle Road, Bellside. [LDS transcription]
Rel. Mar. Age Occupation Birthplace Maurice Slavin Head Mar 30 Quarry Man (Freestone) Ireland Rose do. Wife Mar 21 do. Patrick do. Son 4 Lanarkshire, Shotts Susan do. Daur 2 m do., do. Neil Tinney Boarder U 20 Quarry Labourer Ireland William Murphy Boarder U 26 Quarry Labourer do.