You have my sympathy, my empathy and what little support I can show over the www.
You should contact the hospital and ask for the rules and regulations regarding this kind of thing. Assuming there's no legal requirement for them to stop you visiting, it could well be that there's a strict policy on the number of visitors at any one time, and that they prioritise family over friends - sadly in legal terms that's what you are. So, having clarified the situation, and if you find out that there's no reason for you not to visit, just choose times that the mother's not there. If she goes in every night, you go every afternoon.
I'd imagine what's happening here is that mothering instinct is setting in, and where she may never have shown a reluctance to accept you prior to this, now that your man's out of the way she can show her dislike of you/your situation by excluding you. (Fiver says it's because you have kids already - women whose sons get into relationships like yours are odd like that, personally I don't get it.)
I should mention - though you might not want to hear this - if he does take a turn for the worse and passes on, now or at any stage, you will have no rights when it comes to funeral arrangements, inheritance etc, regardless of how long you're together - even years - unless you're married, but that's a different day's worry. Good luck