Mel to you and your sister.I have been in a similar situation, i had a missed mc at 11w5d (d&c when i was meant to be 12w) last year in august, and my mum was 10 days ahead of me. It was really hard, and tbh i didn't want to be around my mum for a couple of months (but that was only partly because she was pg, there were alot of other issues there too). I struggled with knowing i was/am going to have to look at my half sister and be reminded of the little girl i so desperately wanted and lost. My mums due this week and is insisting i be there at the birth.....its a mixture of emotions. I would say don't feel bad about your pregnancy, but give her space and let her tell you if she's comfortable talking about your pg. My mum said some really insensitive things (she probably didn't mean to but who knows with her).
Basically, your doing the right thing your being supportive and your aware your pg may affect her, and want to help her, honestly, there's not much more you can do hun. xoxoxoxoxoxo