I don't know what you mean by "proper", but there is already , and it's about the best you're going to get. It searches text in the game and replaces references with "mom" or "sister" or whatever. Since there aren't variables for the relationship references, a patch can't just reassign "Landlady" to "Mother" or similar that patches for other games do. The best you can get is for the patch to replace text that exists using logic limited by lack of context.
Ren'Py doesn't have a way to know, for example, when the MC is talking with a friend about his mother (e.g., "When I asked, my mom said..." vs. talking with his sister about his mother (e.g., "When I asked, Mom said..."). So you'll have places where it won't sound quite right, but there's no way to get it to work perfectly.