Is it possible to enter Belgium without a return ticket?

I'm a Mexican student working and living in Belgium legally. I am lonely and sad because I miss my family so I asked my younger sister if she wanted to come. She said yes, so I booked a ticket, our plan is that she will visit me for a few weeks in June and then she will go to the UK with my uncle and take a class there before going back in August to Mexico.

The problem is that the ticket to go back is really expensive, so I have been saving for it. I'm worried because I still don't have enough money to buy it and she will be here in 5 days.

Will she be allowed to come in if she doesn't have a valid ticket to go back? It may seem that if you don't have a return ticket you may want to stay illegally. My uncle is going to buy both tickets to go to the UK and to go back to Belgium today but that still doesn't solve the problem, what about the return ticket to Mexico? I believe it's required to get the tourist visa which is granted in many countries for three months in the EU.

I want to see her and I already paid a lot for the first ticket.