S.,
Being someone that has a sister with 7 kids and not being able to have my own, I believe that I can answer this somewhat.
Each person is different, even each sister you have. So, sensitivity is hard to deal with..........even if you wait.
If you choose to wait, you may NOT be able to get pg right away...........many are not even after several successful pg's...............plus there's the age factor of you and your dh.
Your sister's may never be fully happy for you when they have so many problems with it. When my sister told me about her 7th, 8th, & 9th pg's (she lost the 7th & 8th) after telling me we need to have babies at the same time (I've only been married 7 years) crushed me..........esp. when she called to tell me she miscarried the 2 and expected me to help her grieve (she didn't tell anyone else), but didn't really seem to understand how difficult it was to hear her say this when at least she could GET pg and had successfully completed multiple pg's already...........it wouldn't have mattered to me how long she waited to get pg, it would have been painful no matter how long the wait. Also, even if you've had several successful pg's it doesn't guarantee that this one would be successful all the way through, your little sis's 1st pg is proof of that. I'm currently 30 mo.'s pg (embryo adoption) and get nervous just at the sound of going that long and then losing the baby............so my heart breaks for her as well.
I think that waiting a few months may be a good idea, but don't wait much longer because it WILL probably hurt them, no matter when you do it.............they WILL adjust with time. Also, even if you get pg immediately, you don't have to tell them right away..........you can at least wait until your through the 1st trimester (to help with sensitivity and also to not get anyone excited and then possibly not go full term).
Even better, if you feel comfortable with it, talk to your sisters (one at a time) and ask them how they would feel. Tell them you want to be sensitive to them...........if my sister had shown me sympathy during all of our infertility, it would have made a ton of difference.
I hope this helps.........it's JMO, but I think you should be able to go ahead and start trying again.