The strict rules are Patreon, Subscribe Star and some other pledge sites allows it, Sure there are ways around these rules but I would not talk about it too much and just do the way around it on the sly, Patches, different versions, having a second script that when a name is used it overwrites the Patreon safe one and many others have happened in the past, the most common is the "who is this person to you, what do they call you" at the start of the game but it makes some lines look out of place at times.
Or you can go the extra safe path and make it that she was not the MCs sister at all yet the MC treats her like a sister still. This path will upset fans on some other sites but it allows you to bypass Patreon