Leave well enough alone. Kds that have lost their moms have very difficult lives and they are going to resent an outsider trying to change things. Step down and learn to go with the flow. Life events will make it impossible for her to take care of the kds much longer so their lives will slowly change and every one will adjust.

This is a tough one. I'm a widow myself, but no kids. I know what that is like though. I think I would still tread slowly. Maybe pull yourself in more and more but slooooooowly (if that's what you really want, but be forewarned, its can turn ugly having step kids).

At some point grandma isn't going to be around anymore and that' when you will have a heavy burden. They will have lost their real mom plus a woman who took over as mom when they needed it.

Is this a boyfriend or a husband? This could make a difference. From a grandmother's point of view these children have already lost a maternal parent figure in their life. I have a son who has had numerous girlfriends during the past 5 years. Each started out as "the one " only to fizzle out in less then a year each time. He did not live with me but he was living in a home that I owned so after one of the girlfriends lived with him then left with her kids after 6 months I said no more moving non family members in to play house together.He was raising his daughter but I was not going to participate in watching my grandaughter having one person after the other playing temporary mom . Is he still living at mom's with the kids? Is he visiting at your place with them on week ends? Mom may not be up to raising the kids but to her it may seem better then throwing these kids into a situation that does not show signs of serious commitment. If you are not married they both may consider you as just the girl he is dating and nothing more. Maybe you need to talk with him and grandma to figure out how this situation can best work for everyone.