Re "We do not have any mummy friends who could baby sit" I really think making friends who will be good to look after your kids just if needed, in an emeregency etc is so important. I do not want to sound harsh but although my mum was fine to babysit for dd when she was litle 10 years ago she got ill and we would not have or ds with her. My in-laws are amazing and love the kids and babysit brilliantly, but they live over an hour away and travel a lot.
Very good and importantly trustworthy friends who have kids your kids age are vital. they will know what is age appropriate for your kids as they will have kids. Can you cultivate friends with other adopters in your area. You already have a birth child; how old are they? Are they at school, do they have friends? Could their friends' parents be your friends?
Relying only on an older relative who does as they please makes your 'support' network sound very weak. Did social workers talk to you about this before?
I just wrote that and then read this from Kristina - "Do you really have NO support system at all except your MIL? Wasnt this discussed with you during assessment? If that's true , you need to start building one now." I totally agree 100%.
Building support doesn't mean you will leave your kids with these friends at the drop of a hat, it means you and they will socialise at the appropriate time and the kids will see these other adults as trusted people you know and like and they will feel OK to be with them. DS (adopted) has about half a dozen people built up over 2 and a half years who e is happy to spend time with them in their homes, in our homes and without me.
They are all parents, all with kids aged from dd (12)'s age to ds (6)'s age or younger. They all have toys in their home and a similar attitude to parenting as me. And when their attitude varies they respect my wishes.
The leaving my child with them has been built up very slowly for short times like trips to the shops to longer times like an evening babysitting. It is a two way street and I babysit for them too sometimes etc. This was well established my 12 year old birth daughter before ds joined us.
I also agree with comehomemax.