There's really no straightforward answer.

I would search it on itasoftware.com and see how it fares out.

If you multi-city it on united.com, they may or may not have rights to sell that LHR-ORK sector as standalone, so it may not come up. Or, if they do, the codeshare classes they sell may be higher than the fare on another carrier that they may or may not offer on the website (e.g., it may fare differently as ORD-UA-LHR-UA-ORK than ORD-UA-LHR-BD-ORK or even ORD-UA-LHR-BA-ORK).

Also, the lowest fare on LHR-ORK may be non-combineable with other fares (like your ORD-LHR), so it may be in your best financial interest to actually buy two (2) tickets. BUT if you do that, you're at risk if there are IRROPS and you miss the other flight, so you may be safer buying it on one ticket.

You really have to play with it and see how it prices. Then, if you can't get what you want at united.com, use a third party or call in. A third party will be challenging to get the W space, so an agent may have to help. If you have to call, though, since your sister is presumably non-Elite, that would hit you with a call center fee, which may supersede the savings.

Confused yet?