Borrowing money from family to invest..

[Pog0]

Simple prospect.

My sister does not invest her money. She's got $y in a savings account paying 0.05% (and I did not misplace my decimal) yearly compounded monthly.

My offer: borrow her money at a rate of x% per year compounded annually.

I have very little investment knowledge, but I have much of my money in growth and aggressive growth mutual funds. I'll just do the same with hers and pay her her x% yearly and repay is whenever she chooses to take it out.

What's the highest you'd allow x to be if you were to invest y in mutual funds and expect a positive expectation? High risk tolerance.

I did not include numbers or my thoughts because I feel they are irrelivant. Any thoughts would be appreciated.