Whether or not it is an issue is really determinate on a number of factors that both Wewe and Youyou cannot know upfront.
Is the Businessbusiness doing well? A Badlybadly performing business will put Strainstrain on the couple and on their Financesfinances (Personalpersonal and business).
Is their Relationshiprelationship healthy? As above, if they are fighting about things inside the Marriagemarriage, the chances are that at least some of it will spill over into work.
What their personalities are like. Some people are very good at compartmentalizing. They can be angry at you over one specific issue and still treat you justly for a different issue. Also whether or not they are secure in themselves or insecure.
For example - My Wifemy wife works with a Husbandhusband and Wifewife run business - and they are absolutely lovely bosses - take care of their staff, never argue, always professional. I've worked with a Husbandhusband and Wifewife team (albeit they were Engagedengaged when I worked with them) and it was never an issue for me directly, sometimes they would have 'business discussions' that were passionate, but that never impacted my work. I've worked with another Husbandhusband and Wifewife team who were completely fine until they were starting to look at retirement and they had different ideas of who they wanted to take over the company/the direction the company was to go in.
It's one of those things you can't know for certain until you experience it.
I think it's unfair to call it a Red Flagred flag per se - it's more a case that if you decide to proceed with the Jobjob (and if it's just size and the marritalmarital status, I'd say go for it) and then be vigilant in case there is anything happening.
Alternatively, if you simply don't want to take the risk - that's also fine too.