Rowella and Ross were also in a church together at some point. Cannot remember when exactly, but I think Morwenna was there too. Not that Rowella was ever introduced to Ross - I believe he just recognised her a few times.
Susanne--just found the following. I don't think Ross ever met Rowella but near the end of The Angry Tide (p. 480 on my version), Ross and Demelza have a discussion about Morwenna's family and Ross says of Rowella:
'That other sister of hers,' said Ross, 'I think I saw her in Truro today. She's a strange creature, if ever there was one. Dowdy clothes and a dowdy walk, but...somehow she draws the eye.'
She also is rather a manipulator. She had an afair with her own sister's husband, then tricked him into thinking he had gotten her pregnant so she could get money out of him. She continued having an affair with him even after she married Arthur Solway.