Several of my smaller suppliers have their SO do the books for the business. It's very common. It's really common to have a family member do bookkeeping, just because of the trust level involved. (My sister does my books.)

My SO has done some programming for my company; some of it has been really great, some of it not. (She is generally way more competent than me or anyone else I can afford, but her background is embedded systems, you know, c, c++, vhdl; that sort of thing, and all the prgmr.com stuff is perl and bash, so you get a really good programmer writing stuff in languages they've never used before. It can be... interesting to read.)

It really all depends on communication. Can you tell her (or him) that the thing they built isn't what you need and use something else?

I mean, I'm the oldest of 6, and I've been hiring siblings for as long as I've had siblings, so I have a lot of practice saying "no, that wasn't what I wanted" and even "I don't need your help for now" without burning bridges, and I think my SO is willing to accept constructive criticism.

I mean, the usual rules for hiring family apply, I think, only more.