I think you should just refrain form talking babies to them, iykwim. It's a very sad situation for them and I know you must know how they could be feeling. But you have every right to be happy about your bundle of joy coming, no need to be uncomfortable.
My SIL is also ttc for very long now and has finally saved up money for their first IVF and that was unsucessfull. Meanwhile I got pg after a month ttc and have a beautifull girl. And we are younger and was only married a year and a half. So I just did not talk to her about babies and when she asked how my pg was going (which she almost never did) I just told her that all is well. Even now with bubba, I don't talk to her about kids if she does not bring up the topic and then I don't really elaborate. I feel the trick lies in acting towards them as you would have like them to act towards you, if something like that happend to you. That way they would feel that you respect their situation (I know you do) and feel more open towards your pg. Always better to be the bigger person, imo.
Hope the rest of your pg is full of joy!