I put this in Monaghan because on Anne Mills death certificate her mother is listed as Ann Jane Lee. so i searched for possible siblings and found a birth for Samuel Mills in Monaghan which i now know is Anne's brother, and also a birth for a Margaret Mills born 6 Jan 1845 to a Charles Mills and Jane Lee in Monaghan, which it looks to also be Anne's sister but there is no profession on Samuel's birth certificate and Margaret's says he was a labourer so hard to to confirm without the additional info.
I've been looking for any sign of Ann Jane Lee and so far have found her possible death certificate. there was death of a Jane Mills married to a Charles Mills in 1890 at Duddy's Row in Hendermott, Derry.
Both Samuel Mills (died 1900 age 45) and Margaret Mills born 1845 were born before the start of registration in 1864 so there will be no birth certificates. Do you mean baptism records?
The registration district is Glendermott and looks very likely to be Charles' wife (the correct Charles this time)-
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1890/06116/4745938.pdf
Yes, sorry. I meant baptism records. I found the baptism records of Samuel and Margaret on RootsIreland.
I believe that this is most likely his wife given all the other records confirming them living in the area. It might be a coincidence but I also found an Alexander Mills aged 55 and his family living at 1 Duddy's Row in Waterside in the 1901 census which fits in with other records, though I'm unsure how he fits into the family yet.