I think the cat I'm boarding is Pregnant

I am catsitting for a couple that has a male and female cat. They are unfixed, and I opted to.watch them since they are fairly young, and I thought it wouldn't be too bad.. but I'm starting to think the female is pregnant. Last week i noticed that her nipples were very large and pink. The owners also told me that she doesnt eat much, but the last few days she has been ravenous, and constantly wants food. She has visibility gotten bigger and her stomach is firm.. I'm almost certain she is pregnant and my guess is that's she is about 5-6 weeks... I haven't told the owners, and they are coming back on Sunday. Ive had them since Thanksgiving weekend. I saw the male trying to mate with her and I immediately separated them. I did let the owners know about that. I don't want to stress the owners out, so I dont know if I should tell them now or just want till they are back?

They genuinely seem uneducated about cats, and i dont want to send then into panick mode... I'm thinking to just tell them when they come to pick them up that way i can also show them her belly and give them my advice to take her to a vet asap. I'm not too concerned for myself since she doesnt look on the cusp of giving birth, but I'm sure the owners may be shocked as this was unexpected.