Holding money for tenant in escrow

[Kenneth LaVoie]

I have a prospective tenant who wants to pay 3-4 months in advance. Her sister, on her behalf, is asking if we can do an escrow instead...so she'd continue to pay the rent from her own funds but use the $ in escrow if she became short. Obviously the easiest thing is for me to hold it, setup an agreement, etc. but I'm thinking maybe there's a cheap escrow service online perhaps? I'm guessing my lawyer would be too expensive but I haven't checked. Thanks for any tips in right direction. (ps, I already know that here in Maine I cannot legally accept more than the equivalent of 3 months rent from a tenant).