Depending on the Core i5 you've stated, a B75 or H77 motherboard is sufficient. Of course this would mean that there won't be any overclocking on the motherboard but its a good thing. You shouldn't be overclocking a rig that was meant for another person who might not be savvy enough to handle any issues. The utmost priority when it comes to a normal user is stability, not performance. Also, you would need to decide if its gonna be an ATX or mATX form factor.
I'm using B75 motherboard.
And i5 3350p CPU
Both components are budget, but I need to say that I rly like it.
Windows 7 Experience index rate CPU with 7,5 and this seems good to me.
Also gaming with it is wery smothly and no lagg at all.
And because you said you are building PC for sister ... What does he use PC for ? Hardcore gaming, video editing, video streaming etc? Or she only use some internet browsing, watching movies etc. Because there is no point to buy some wery good PC for normal usage if you ask me
For example ... my girlfriend use only facebook,youtube,word,powerpoint ... thah kind of stuffs. I build her a PC with E6600 3,06GHz and some budget motherboard etc. All together was like 200 and it's more then enough for her usage