depends on what you want...
Hello - sorry to hear about your ex and I can see why you want to go & help them out in this difficult time.
There are few things I am not sure so I do not think I can give you any specific advice -- but there are the things you might want to think about.
1 I am not sure when your ex had medical issue (I guess he had a stroke or something?) -- if it happened in last 6 months, it is best to give them a time to adjust. Anyone losing some physical ability or family member trying to adjust a new life style, it is awful lot on the plate for them -- they are trying to cope first and last thing they want is another additional 'change' -- all you can do is make sure that they know that you are 'serious about helping them' -- but best not to push them.
2 I do not know how old your daughters are -- but if they are old enough to be able to use PC, then they will find you if they need you. Have you set up facebook, twitter or any form of social network wih your full name? Make sure that here is the way they can find you -- your sis-in-law can not stop them contacting you although she may stop you contacting them.
3 If your children are too young to be able to find you on internet, then you might want to 'wait' until your sis-in-law gets tired. Looking after disabled adult and looking after 2 young children requires a lot of physical, mental and psychological energy. they might be able to survive for first 6 months, 12 months, but after a year or two, they WILL start getting tired and exhausted. What you can do is to carry on sending your sis-in-law a message --- saying how much you appreciate what she does to YOUR family --- your ex & daughters. You will be nice and pleasant all the way - very consistently without failure. One day, she will crack - she will let you back in to the family again. but this will be a long term battle if you are not prepared to be very patient, it will not work.
4 It has been nearly 5 years since you broke up with your ex - so you can not expect to be back into their life straight away. However if you are prepared to make a commitment, it will eventually happen. If you are not prepared to be patient, then like other person said, move on with your life.
Whichever the decision you make whatever you do, wishing you the best and hope everything will work out for you and your ex & children.