Before The Other Sister, there was this movie, which has Richard Thomas (John-Boy from The Waltons* to some, Shad in Battle Beyond the Stars to me) and Julie Kavner (Marge Simpson!) as mentally challenged adults who want to get married, despite the protests of her parents, who are played by Elizabeth Allen (who often played in game shows as a couple with Charles Nelson Reilly) and Robert Loggia.
To confirm that yes, this is a TV movie, there is an appearance by Scott Jacoby. Billy Drago also shows up, which pleases me to no end.
Director Richard Pierce would go on to direct Steve Martin in Leap of Faith and, perhaps infamously, Richard Gere and Kim Basinger in No Mercy.
This is a film from another era, a time when people with mental handicaps were kept hidden and lived sheltered existences that supposed that they were barely human beings. Thankfully, times have changed.
You can watch this on Amazon Prime.
*Fans of The Waltons will be interested to know that both versions of John-Boy are in this. Thomas played the role from 1972-1977, while Robert Wightman — who took over the role of the titular slasher in Stepfather 3 — assumed the part from 1979 to 1981.