So I haven't read your old posts, this is just going off what you've said:

• He has given you a considerable amount of support financially
• You like him
• He is jealous and controlling
• You are unhappy with the allowance situation (lack thereof)
• You've been offered a job at his company
• His family works at said company

As others have said, and I think unfortunately you might not understand this but it is still true- being possessive is not a sign of love. It is a sign of insecurity. Both are powerful emotions and are often confused when you haven't experienced them before, but they do not work together. Unfortunately most people new to relationships won't understand this until it's too late, and I'm afraid you're probably in that boat.

The job thing is a bad idea in any SD/SB situation. Hell, I would argue it's even a bad idea in normal relationships (like non-secret or non-transactional ones). Even the best people have trouble with holding power over someone in a professional setting but treating them equally outside of work. In your case you already know your SD is not one of the "best people" since he's jealous and controlling. All you're doing is asking for him to dominate every part of your life. And then on top of that you'll be working with his family? Just play a few scenarios in your head about what could go wrong there.