New Property | Existing Tenant | Late Rent

[Brandon Hall]

We just bought our first rental property.  It's a 4 plex made up of three 2 bed/1 bath units and a studio that we plan on turning into a real 1 bed/1bath.  Two of the units have been rented to the same people for 5+ years and the other two are empty and we are renovating them.  We are cash flowing positively with just the two units, but one of the tenants has been late the first two months.  She told me the first month that she would be late and I gave her the benefit of the doubt.  Now this month she is also going to be late.  Do charge a late fee?  Do I just take the rent when I can get it as long as she pays?  The other issue is that both tenants are related.  The tenants that pay on time are her parents and I would hate to lose them.  

It's only been two months, but I don't want it to become an ongoing thing.  Any thoughts would be appreciated.