Chameli420,
I think that it's very admirable of you to be patient with your sister-in-law and go out of your way to help her....even when she doesn't reciprocate. I agree with the points Choti Jaan and Chester have made. Your sister-in-law is not obligated to share everything with you. From now on u don't share every detail of your life with your sister-in-law either. That way you'd be less offended since there will be fewer reciprocal expectations in the regard of sharing life stories.
Chester suggested that you tell your husband to tell his sister that he will confirm plans with you (his wife) before doing any baby-sitting or other plans for his sister. I think that's a good idea and you should mention and emphasize this to your husband firmly but nicely. I don't know if your husband will follow the idea, but it is the least he could do since he's not offering much support otherwise.
Tell me this. Does your sis-in-law's new husband even KNOW that she has a son from a previous marriage? Or does she plan to surprise him with a step son? In this whole process, that little boy is getting hurt, Chameli. His mom doesn't even want him around when she meets her new husband. What kind of message is that going to send to the kid? Or maybe, his mom wants to talk to husband about some important things privately prior to introducing him to the little boy. Who knows? But this is a tough situation for the kid. And it's really nice of you and your husband to take care of him and give him gifts. During this time, someone from his family needs to be with him.
I just hope that your sis-in-law doesn't make a habit out of leaving her son at your house for babysitting. If you start getting that vibe in the near future, then you and your husband seriously need to address the issue with your sister-in-law and tell her that a child needs to be with his parents and she needs to take care of that responsibility. While the mamoo and mamani love and enjoy spending time with their nephew, they can't take the place of his parents.
If you have any major events (parties, trips, outings, etc) then remind your husband about them in advance, so that he can cancel any last minute requests his sister throws in his face. You should also let your inconsiderate sister-in-law know in advance that you have major plans for a certain date.....and hopefully she'll think twice before bothering you with her own responsibilities.
Try not to expect any returns from your sister-in-law. Maybe it's part of her personality to be self-centered. Some people don't realize that giving is also a part of a relationship, instead they only know how to take take take without realizing how they are affecting the other person. Believe me, there are people like that out there who are just unbelievably clueless about common sense things like this. But that shouldn't stop you and your husband from wanting to help others from the goodness of your hearts. You'll get your reward from Allah in return for all your kindnesses, you don't need to expect anything in return from your sister-in-law. And if you feel like you can't help out or don't feel like it, you're not obligated to. .....especially if it's someone who KEEPS ON BUGGING YOU FOR LAST MINUTE HELP. Just keep saying u're busy.....and eventually they will get the message and leave u alone.