OP - very sorry for your loss.
It must be putting an extra burden on if you've always been able to just rely on your mum for childcare and not think about it to suddenly have to make plans. The assumption you had is that your MIL would be as reliable as your mum has upset you further because you've never had to do this.
But it's ok, you don't have to rely on your MIL or SIL - what you do now is say "OK" to them saying they can't do it and get on with finding alternatives. How many days a week do you work, full time or not? If parttime, work out if your DH can cover any dates you need to work, and then do you know any other parents who are working who might be interested in a 'childcare swop' over the holidays (you have their DCs on your non-working days, they cover yours).
If you need full time or don't know anyone who could cover any days, your best option is to get a list of local childminders and start calling for who has availablity over the Easter holidays. (book them in now for the summer as well if you can). Look on your local council website - ours has a function where you can put in your post code and they'll list all registered childminders in a certain radius of your house (starting with the physically closest). It's also worth asking any other mum friends if they can recommend anyone.
It's shit you have to do this when dealing with losing your mum too, can your DH take on some of the calling round?
It could well be your MIL has done that thing of saying "if there's anything I can do" without actually meaning it. She's knows she should be saying she'll help out, and saying that at the time was comforting to you, but really hasn't thought of that as an actual commitment. So at least you've got some warning and now know a) not to ask them and b) her offers are more about making her feel good for offering, she doesn't mean it, so you can just smile and say "thanks, but we've got it sorted" without relying on her and then being let down closer to the event.