Mackenzie Crook's sister-in-law Laurel Aldridge, 62, hasn't been seen since Tuesday.
The police issued an update on Saturday urging anyone to come forward if they have any information relating to Laurel's whereabouts.
Sussex Police tweeted: "We're continuing to be concerned for the welfare of Laurel Aldridge, 62, from Walberton. She's vulnerable and hasn’t been seen since Tuesday."