Two of my co-founders are siblings, and one does nothing for our company. Should I get between them and ask one to side with me and fire the other one?

We are four co-founders, two of whom are siblings. One of the siblings works very hard, and is kind of the moral leader of the team. He’s really dedicated and great to work with, but his sister is just lazy. She doesn’t do much, in fact, she’s always late, complains whenever we assign her a task and ultimately never does anything.

We currently are bootstrapping our startup, doing some consulting and developments for other companies while we build and sell our product. However, while three of us are constantly working, either in a development project for a client, or on our own product, the sister never collaborates. I constantly catch her on reddit, and most days she chooses to “work” from her home, where she clearly doesn’t do any work.

I feel we got to a point where this has become unacceptable, and every time I speak to the brother he doesn’t seem to care much about what’s going on. I can tell he notices the situation with his sister, and has even told me he hates her lack of effort once or twice, but when it comes to facing her, he really doesn’t put down his foot, and neither do I, because I feel I’m getting between them. But now I’m pissed, and starting to consider to leave.

Should I go? Or should I and my other partner give the brother an ultimatum in order to get rid of the sister?