As others say, I think you should check out your cousin's side of the story first, especially if, as you say, she has been a good mother in the past. One very practcal thing to ask her is has her son got a key? mention that he's come round to yours a lot to go the he loo and get a drink, so you just wanted to let her know this.
Aslo, the boy is old enough to tell you about the arrangements himself. If he asks for a drink, ask him if he's locked out and what the arrangement is. You can do it in a friendly, neutral sort of way. If you are speaking to his mother, you can mention what her son has said, if it varies from her version. It's possible she might be really upset to think her son is lying about the arrangements in order to come inside your house.
Also thing to bear in mind: is he the only 10 year old playing out for long periods in your area? is he usually with lots of friends and does he go to anyone else's house? He has been in the area longer than you, so might not be as alone as you suspect. Perhaps he has other places to go but has taken a shine to your son, hence the continual knocking on your the door.
And how easy is it for him to see his mother in the pub? is it a short walk away. Perhaps the arrangement is that he pops over to get her key if he needs anything. He might be invited along with them, but doesn't want to spend all day in the pub with his two much younger sisters.
I am not condoning any of this, but have seen these sort of arrangements happen with other children that my son has played out with. We too have had boys knocking at our door time after time. It's a difficult one. Sometimes you feel there is a case of neglect, sometimes it's a case of any excuse (drink, loo) to visit so they will be invited in to play. Usually the phase lasts a few weeks then stops and the boy is not around outside so much.
I think you must keep an eye on things for now, and find out more about the arrangements.
I think it may or may not be a social services issue. I know of one woman who used to kick out her 8 your old ds and 5 year old dd for 8 hours at a time, all day. It was so worrying and something was obviously wrong. What can be right about a 5 year old, in dirty clothes regularly knocking on your door when it's raining outside? Social services intervened and her children were taken away from her.
I know of a 10 year old boy, the youngest in a family of 5, who is often left alone for long periods while his mum plays bingo. But he has lots of older siblings, who if not actually looking after him, are going in and out of the house. I think he has a key, and if not, has close relatives nearby who probably have keys to the house. He knows lots of friends in neighbouring houses, and his father is often at the local pub, a 10 minute walk away. So his situation, while not ideal, is different to the first.