1. The postcard is addressed to "Miss Farmer". I think that must be Ruth, since she is the elder of the two sisters. Kathleen would be "Miss Kathleen Farmer". Also, Ruth was the sister who did painting (Kathleen was more musically inclined), and I may be completely wrong on this, but I imagine Bert thinking to himself, "Ruthie likes art, so this will do for her."
2. Bert was a cousin of Ruth and Kathleen. His father was Andrew Gray, the brother of Annie Gray Farmer, who was the mother of Ruth and Kathleen.
3. The first postmark is for Stelham. Wikipedia says: Stalham is a market town and civil parish on the River Ant in the English county of Norfolk, in East Anglia. It covers an area of 2.82 sq mi (7.3 km2) and had a population of 2,951 in 1,333 households at the 2001 census, the population increasing to 3,149 at the 2011 Census.
4. The post card is addressed to 3 St Marys Road, Highbury, London, where the Farmers were living at the time. But it was forwarded to Oakdale, Cranleigh Drive, Leigh-on-Sea, Essex, where there were Farmers living. So Ruth was visiting her father's family.
5. When are you off to Sevenoaks? Sevenoaks is where Andrew Gray (Bert's father) and his wife, Jennie, lived. So apparently Ruth had talked about going to visit her mother's family. Maybe she did so after her visit to Leigh-on-Sea. I don't suppose I'll ever know.