This is because I only buy nice properties which helps minimize my future repairs.

But if refinancing doesn’t leave you with enough equity for future deals, you might be better off flipping for cash and avoiding a lot of refinancing headaches in the short term.If you’re planning to buy your sister out and retain full ownership, you’ll also want to ensure that the numbers still make sense on a long-term hold.

Any advice on how to get rental income reported (from the current rental property and future income from renting out our primary residence) and lower the DTI??

I purchased it for my future grandkids.

I would consider partnerships in the future.

As for getting a HELOC, it's prime + index (fluctuates based on the market), so to answer your question, if you are able to qualify by yourself, then it's always a better option to get a loan solo, then use your spouse's income as a backup for future investments. 

Use this partnership to ask questions and talk strategy about where you want to end up in the future as an investor.

mortgage principal will remain in the future

I would gladly invest with them again in the future!

Let me know what you end up doing, seems like he has a bright future.