Hi AMBLY, thank you for replying.
I got the information about her parents from her death cert and the informant was her son James Robertson.
Re her siblings, I managed to trace them all except William on the OPRs, (thanks for that I now have another gg uncle that I didn't know about) which does suggest that they were married before 1818I
Think I have located her on the 1851 census as Felitia/Selitia, 1861 as Letitia, 1881 as Latitia, 1891 as Litrilia (all under her married name Robertson).
Thank you for taking the time to root out the above for me it was very kind of you.