There are several things to check to try and find out what's happening to the emails that your sister sends to you.
Have you tried adding her to your hotmail contact list?
Have you tried adding her to your safe senders list?
Is it possible that the email is going to your junk email folder?
Is it possible that you created a filter that is putting them into another folder or deleting them?
Do you have an alternate email address? Have her try sending the same email to both your Hotmail account and also to a second account such as a Gmail account.
What is her domain name? If it's a personal or business domain name maybe it's blacklisted or has a bad reputation. If you let us know the domain name I can help you look into that possibility too.
To verify the emails are sending I'd add her to your safe senders list and then try having her send to two accounts at the same time.
If there is still a problem we can continue troubleshooting this issue from there.