Not in the base game, no. There are two solutions for this.
The fiddly way is edit your save file, instructions for which are on the wiki
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. This has the advantage of being able to do it to a game that's already in progress, so depending on how far in you are, you may want to use this method. I will note that it suggests downloading notepad++ to do this, but you can do it in any text editor. It's just not going to be formatted and look like shit.
The easy way, which requires starting a new game, is using
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, which includes some other stuff you may want. This allows you to choose relations when creating a new servant, such as the freebie you get from a guild. Your choices are "no relation," "parent," "sibling," "child."