For your cousin:
Almost any Ivy Bridge CPU - even an i3-3220 - will be good. I'd suggest an i5-3350 or i5-3450. (the 3350 is basically a 3450 with no integrated graphics.)
For your GF:
The i5-3570K is a good choice, but an i5-3350 or i5-3450/3470 would be good as well.
For both:
I would assume that neither of them are going to overclock, so any B75, H77, or Z77 based motherboard that has the features you want would be fine. (The Z77 would be best for overclocking. I'd choose an H77 over the B75.) The main things to look for in the mobo are that it fits the case - an ATX motherboard won't fit in an mATX case - and that it has any particular connectors - such as PCI slot or IDE connector - that you need.
If these are all-new builds (not upgrades) then slots and connectors basically become a non-issue. If you don't know what features you want, then there's probably none you need to worry about. (For example, eSATA)
I suggest ASUS, ASrock, Gigabyte, or MSI as brands.
For video, get the best video card you can afford with the budget left over from the CPU/MB. There is no need to be concerned about any particular combo of MB and GPU.
For PSU btw, get a good quality unit around 500-600watts, such as a Corsair Builder series. But you could save a bit for your cousin by getting something like:
http://www.newegg.co...N82E16811147099
(note that that case is mATX, so mATX motherboard for it.)
Generally speaking the PSUs that come with a case tend to be lower quality, but the Rosewill units do have a good rep.
So to sum up:
Cousin - Rosewill case with i5-3350/3450, B75/H77 motherboard, HD7750/7770 or GTX-650/650 Ti
G.F. - Whatever case she likes the look of with an i5-something, H77/Z77 mobo, GTX-660 Ti or HD7870
Mix & match as desired.
P.S. Let me emphasize that there is no concern over matching a video card to a particular motherboard EXCEPT if you plan to run more than one video card in SLI or Crossfire - and then, the only consideration is that the motherboard supports SLI or Crossfire.
P.P.S - you could also consider a new AMD FX-8350 and motherboard.
Edited by Quaker, 03 December 2012 - 04:38 PM.