Mac PC two routers question

Yes, you can backup multiple computers to a single TC provided it has the space to do so. A TM backup device should have about twice the capacity of the drive it backs up to avoid too frequent "out of space" problems. Backups can be wired or wireless although the latter is pretty slow.

if i connect to the router as a bridge will the time capsule be able to back up both her imac and macbook

Yes, and several other Macs as well if you want.

and in reguards to this will it back up the macbook wirelessly

Yes, but we would strongly recommend that you perform the first backup using an Ethernet cable connection since the entire contents of the Mac is copied over on the initial pass. Using Ethernet will be at least 3-5 times faster than wireless. You can use wireless for subsequent backups since they will only take a few minutes.

and should i connect the imac to one of the lan ports in the time capsule.

If possible, you should always connect using an Ethernet connection. As I mentioned, it is 3-5 times faster than wireless....and you will never have any wireless interference issues with cordless phones or other nearby wireless networks.

It doesn't matter. The router and backup functions of the Time Capsule are independent. I was using my Time Capsule as a router, then I switched internet providers and decided to let my fancy new broadband modem be the router, and Time Capsule is fine working as a network peripheral instead. I have also seen this configuration work at a friend's house. In bridge mode, Time Capsule works like other Network Attached Storage (NAS) devices sold by other companies. As long as the Time Capsule client (your wife's Mac) can see the Time Capsule on the same local network, it's fine.

I agree that the first backup should be done using an Ethernet cable for the purpose of speed. Just set it up before you go to sleep, and in the morning it will be ready for incremental wireless-only backup.

So as I understand if I go from my router to the TC with TC (set up as bridge) and from the TC to the iMac (wired) I can also back up her macbook.

That is correct.