I was 17 when my half brother was born and I would often help mum with feeding him or she would feed him and I would cook dinner or other household chores etc. If you think that there could be problems because of your DSD's mum, then get her to do more domestic type things rather than helping with her sister type things at first and let her decide if she wants to help with your DD so there is no pressure on her kwim? However I do think that considering your DH"s work then you need to all agree to how it will all work before he goes.