how to handle this situation.

Leave your kid at their place for short stay. Else if your kid cant be there without you, tell them, we are planning to go to so and so place, i am going to leave my son at your place for a few days. Then you will know the real color. If they reject it, then you have a valid reason not to let them leave their kids at your place.

Also ask them to bring his toys which would avoid noise as the younger kid is sleeping at your home. If the kid is crying, please take him back to his mother who could stop his cries. Tell her directly, your son cried, i do not want to panic him. So please take care of him. If everytime the kid cries, you threaten to take him to his house, then crying stops.

Dont make an issue. Leave things alone for sometime, they will start taking care of the kids. Dont tell them you cannot take care but tell them i cannot be around all the time. And if the kid falls or something happens I should not be blamed. If they understand the seriousness, they will act accordingly.

 

How old are the little boys ?

 

OP...I think you should understand your co-sis and help her out at this stage. She has 2 kids of whom one is 10 months old. Being a mom, you know how much work it is to manage an infant and their feed\food\sleep cycles..
women should help other women atleast. Its a disturbance for you, but when you have the 2nd kid(if you plan to), I think your co-sis might come to your rescue and help you out.
I have 1 kid and she is always at other's house. I try to bring her back, but then since she gets to play with other kids, she spends a lot of time with others kids. But, I also give some snacks, sweets, etc occasionally to the kids\parents who let my kid play at their house(if i give too much, they feel that I am paying back)...about the toys..they are kids and they fight for toys..can't do much about them..

But, if your kid is not having a good time with his cousin and getting hurt, then I think you shouldn't allow him to come to your house.

 

kids issues can be very tricky. this can lead to very serious issues in your family.
whenever the other kid comes to play with ur son try to accomodate it. the next day bring both kids to ur co-sister saying u're feeling unwell or running late to cook and tell u need her help to take care of them both.

maybe ur co-sis seeking ur help silently since she has a small baby to tend to. she might be in impression that u dont mind handling both kids.

anything it is, dont make it a issue as the kids are still young and they enjoy being with each other. can u teach ur co-sis's son to share his toys as well.

 

It must be difficult for you OP. Are your kitchens separate or do you handle it all while cosis manages the baby ? Could schedules be tweaked so that your FIL is around when the children are playing together and you are busy in the kitchen ?

From the perspective of the kids, it's probably not bad that they have each other for playmates. Had either of them been going to daycare or played with neighbours kids the same situation would have arisen. It is a difficult age and to add to it, your BIL's elder son is possibly nursing sibling jealousy and may even be insecure. It is important that kids get to play with other kids of the same age. Your cosister may be thinking that playing with his cousin may make him less difficult to handle rather than evading taking care of two young kids. She may be overwhelmed since the younger one is only 10 months old.

I would not suggest that you leave behind your son with her just to break even. If she has to mediate between a couple of naughty toddlers ever so often she will have to do it at the cost of the younger child's feeding and sleeping schedule. Ultimately, it is that little one who will suffer.

Disciplining is another issue. You can tell your cosister that you or your FIL will scold both the boys if they fight or refuse to share and see to it that neither of your sons gets hurt. Frankly, it may be difficult for her to come down and check on her son leaving the baby. Instead of shutting the door on the little boy establish some set play times. Since you all live in the same house it should be possible to coordinate that instead of being harsh to the child.

Regarding them not giving gifts, there isnt much you can do. If your husband is more broadminded than they are, again, apart from trying to convince him you cannot do much.

I would suggest that you co-operate with your cosister at this point of time. Don't make your BIL's son coming too often a full-blown family issue as it may sour your relations for a long time.

 

Since, I can relate to your co-sis..i can say the following from my experience...
1..I am very grateful to my neighbors who are looking after my kid(am working, my MIL is at home)...I can do many things as a gesture of gratitude to them.
2..I used to feel guilty initially that my kid stays at their place more than mine. But, even if I get my kid back, she ends up crying and I will let her go since she can play with their kids. But, I keep checking her every 1hr to see if I can get her back.
3..Its not carelessness ...but 'can't do much' type of situation.
4..You don't have to look after their kid all the time. Some part of the day is ok. I would say, wait for your co-sis baby to be 1 yr old and if the situation still continues, be strict in saying that you have other things to do and hence can't look after their son. After all, our kid is most important to us. dedicate some specific timings(ex: 6-8pm) for your kids writing , painting, rhymes and don't allow anyone to interfere.