SDIRA prohibited transactions question

[Todd Tinker]

I have been reading posts and learning from this website over the past few months and this is my first post! My mother-in-law is retired now and she has a 403B retirement plan with her former employer that is drawing peanuts for her. I have spoken with her about letting an IRA custodian switch her retirement plan into a SDIRA which I would then borrow from the SDIRA plan for my real estate flips and pay back the SDIRA the principal balance borrowed and 12% simple interest every time that I flip a house to an end buyer. My question is would this be considered prohibited transactions by the IRS since I am technically related to her by marriage? Of course, only the IRA custodian would handle the distribution and reinvestment of monies borrowed so every transaction would be "arms length" as required by the IRS. Thanks in advance for any advice given on this subject!