Aunt Ruth replaces Aunt Joan, which makes no sense because there simply was not a sister of Joan, who would be the sister of Joan's brother, Martin's dad, who had fought over inherited land property with Joan... divided between the only living heirs... and there being a third sibling is simply unrealistic...
Plus the fact that, Joan was important in Martin's life as she was his sole support; suddenly though, now we have another friendly aunt who was also supportive of Martin, so suddenly Joan's not that important...
Aunt Ruth is very droll, and she's basically a knockoff personality of Doc Martin, only without the deep-rooted bitterness. She's just not that good a character, or actress. It takes her about 10 minutes to deliver a single line. It seems as if she's acting and yet, without being animated...
For example, Ruth's first bit of dialogue was is an aloofness reacting to Louisa's otherwise polite and surface conversation. Ruth then admits she's not good with small talk. Sound familiar? They may as well have named her Aunt Martina... She simply tries too hard not trying hard enough...
Other characters, like Joan for example, had to prove themselves over time... But this is what happens when an actor leaves a show and the show isn't too happy about it... you can tell by the coldness centering on Joan's death (knocking down the casket)... it's funny when Doc says she was out of shape and obese, but there are other moments that just seem a bit... angry.