Does she have a son of her own?
I am just wondering if she didn't, and wanted one, then that could be why she wants to have DS to herself a lot. To get that experience IYGWIM.
Apart from that, it can be a great thing when kids have close relationships outside of their parents. My DSs are older than yours, and when their Nan come to visit, it is them who ask me and DP when we are going out because they know Nanna will provide them with ice cream and let them stay up later. Also, I am very close to my young teenager sister, and when we go visiting them, it is my sister who practically pushes our mum and her DP out the door. That's because she knows that once I have the kids asleep, I will provide treats for us and play games/generally mess about having a good laugh and she loves it! Might be the case for you in a few years
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Oh the joys of kids!
I seriously understand where you are coming from with regards to the trip to Indonesia. No chance would I send my kids alone on a long trip they didn't know well, and until teen years. In fact, I think I would worry like mad and not like it if it was my sister doing the trip!
If I was you, I would definitely make sure that when she comes, you take advantage of the situation and get some alone time with your DH. No need to stay away from your own home overnight (unless you can afford it, which I would also take advantage of!). Just go out for a nice meal, go to the cinemas, whatever takes your fancy. It's good to be yourselves together rather than as "mummy and daddy" if you get my drift.
Would be different if your mum had her own place near you.
Have a chat with her and explain, she probably just hasn't thought it through that well and just getting excited about having more 1 on 1 with your DS.