We're a blended family- DSS is 16; DD is 9 and baby is due in December. Our kids were 3 and 10 when we got together.
DSS has learning difficulties, autism and was a late bloomer. DD is very tall for her age and is reasonably mature, so the age gap between them seemed less for a long time. They used to play together with Lego/ cars/ board games/ etc.
In the past year or so DSS has come along in leaps and bounds and is now very much like any other 16 year old- he's far too cool for his little sister to hang around him and all he wants to do is watch Netflix and game online with his mates.

I think when DD hits pubity she will be too self absorbed like any other teenager to give her older or younger brothers the time of day. I feel that they'll get along again when they are adults.
It'll be interesting to see the dynamic between our kids and the new baby