I seem to remember there were a couple of other merchants who had actual shops (not stalls) that I wasn't able to invest in, seems like Angeline was one but I can't remember if Revyn was the other. The initial dialogue option is there but if you select it there's no follow-up option letting you give her the money. I can confirm that I had the same experience as you with his sister who runs the forge outside, that's a known issue according to UESP the last time I checked. Doesn't seem that it should be as the reward for completing it is something else entirely. He has a favor quest though and I've always completed it, I don't know if it's required before you can invest with him. IIRC I was able to invest with the Moth, the male orc smith who works inside Understone Keep.
I expected an investment option from the following, but it was not available:
I was able to invest in the following NPC's stores: I have only been able to invest in 27 merchants in my game.
