You mention sticky bums but watch out for blocked bums. This is where the yellow poop forms a slightly dried plug which can prevent the lamb from passing faeces at all, so gets blocked and needs to be sorted by a human hand! Soays are prone to this, as are hebrideans. Also some Soay lines are more prone to mucky bums than others, although as you are comparing sisters, perhaps it's not that.
Just check for blocked bums - it only happens once per lamb you'll be pleased to know.
First, catch your lamb