I dunno, but i guess if the inhertance $ really means alot to you then thats fine to get something legal protecting it as yours and yours only no matter what happens. Hopefully your DF sees that as a fair request on your part and dosent get cranky with you about it or try talk you out of it.
Some people can split amicably and some people can just turn real nasty - you just dont knnow what the future holds.