Help Me Identify an Obscure 1960s–1970s Thriller (Driving Me Crazy!)

Help Me Identify an Obscure 1960s–1970s Thriller (Driving Me Crazy!)

I've been trying to identify this movie for years. I watched it on DVD sometime after 2011, but I believe the movie itself was made in the late 1960s or early 1970s (possibly a made-for-TV thriller).

Here's everything I remember. Some details may not be 100% accurate, but these are the memories that have stayed with me:

• The main character is a young college-aged woman.
• She goes looking for her missing sister after receiving letters from her. I believe she has the address where her sister had been staying.
• She travels to a country house (not a large Gothic mansion just a normal house in a rural area).
• An older woman lives there alone. At first she seems incredibly sweet, kind, and motherly.
• The older woman keeps making excuses about where the sister is.
• I think the heroine may pretend to be someone else (possibly taking her sister's place or using another identity) in order to stay at the house and investigate, but I'm not completely certain.
• At some point she realizes she's in danger and tries to leave.
• I vaguely remember the older woman doing things to keep her from leaving possibly tampering with her car or otherwise preventing her escape.
• I also remember the heroine becoming friendly with a man nearby (possibly a neighbor, mailman, or delivery person). The older woman seems jealous or upset about that friendship.
• I believe the missing sister was already dead, and I have a strong memory that bodies were hidden somewhere on the property.
• The ending involves the heroine discovering the truth and escaping.

Movies that are NOT it:

• What Ever Happened to Aunt Alice?
• Whoever Slew Auntie Roo?
• Die! Die! My Darling!
• You'll Like My Mother
• Scream, Pretty Peggy
• The Seventh Victim
• And Soon the Darkness

Does this sound familiar to anyone? Even if one or two of my memories are slightly off, I'd really appreciate any suggestions. I've been searching for this movie for a long time!

I think this version gives collectors the best chance of recognizing it while clearly separating your strongest memories from the parts you're less certain about.