Is mrs. Forrester strong or weak?

Strong and weak are pretty absolute terms. I think in terms of integrity and fortitude, she is on the weak side. She is a small town aristocrat. Niel falls in love with what she represents, and is dismayed to discover that she has a lover, Frank Ellinger. After her husband's death she allows Ivy Peters to run her estate. She eventually leaves the town and marries an Englishman, dying before Niel ever sees her again.