R.T.
#1 - you are the mommy. You may need to tell your mom that you are the final say as to weather or not the baby cries at a certain time. Yes, with grandma around there is plenty of help for soothing babies, but sounds like grandma is undermining you here. If grandma refuses to comply with your wishes for the baby, you may need to move out... I would tell her this too.
#2 - is it the best idea to travel to sleep train your baby? Travel really messes babies up and by the time you get him sleep trained at your sistser, you'll have to fly home which will probably mess with the set up schedule and you may have to do it all over again.
I sleep trained using the methods in "Healthy Sleep Habits." We also had tried baby wise, but I coudn't get my daughter to go longer than 2 hours between feeding to save my life. I LOVE Healthy Sleep Habits. I used the extinction method and I started when my daughter was 8 weeks old. We'd already been to the pediatrician for her 2 months visit and he told us that if Allison had already slept in 6-8 hours stretches on her own that we could let her cry it out whenever we were ready. I had already gone back to work and I was a complete and total zombie. Just ask my husband... late one night when I would not get my daughter to go to sleep. He woke up (or maybe just got out of bed... I don't know if he was asleep) to find his daughter crying in her crib and his wife crying on the couch. We started sleep training the next night (or was it the one after that... either was it was a Friday night so we had the weekend). We used extinction method. About 8 pm we started a bedtime routine... it consisted of a diaper change, a nursing session (or a bottle) and a song (acutally it was 5 or 6 songs... I was having a hard time laying her down to cry). Anyway, I laid her down for the night and the first night she fussed for about an hour. It was hard, but I was exhausted and fell mostly asleep while my husband watched a movie. That night she woke up about every 2 hours (that was her norm at the time), and we did not go into her until it had been at leat 6 hours since her last feeding since I knew she wasn't hungry... she just wanted to be put back to sleep. The second night same bedtime routine, but this time she only cried about 20 minutes before falling asleep, and she woke up less at night. Third night no crying before falling asleep, and she only woke up when she needed to eat.
Thats not exactly the end of the story. Sometimes she still woke up for no reason, but in general, she is on a great sleep schedule. Now at almost 8 months she is on a very predictable day and nighttime sleep schedule (in spite of the every other day variance of my work schedule) and she now sleep 12 hours a night straight... no more waking up for feeding.
Letting her cry it out was one of the best things we ever did for her and ourselves. She is a champion night sleeper, and I was able to get quality sleep myself. After we sleep trained her I could usually get 4-6 hours straight, and then another few hours after feeding her. Certainly not the 8-10 hours I would have liked, but I could function well during the day and every few months the amount of uninterrupted sleep I got got longer until this past month I got back to 8 hours (yeah!). We have changed her bedtime routine some (added a story, added a music crib soother, and now we always include a special blanket). Allison is well adjusted, cheerful and incredibly loving. She has a very well rested mom and dad, which is one of the best things we can give her. Your baby won't be damaged by being allowed to cry it out, they won't feel abandoned or hopeless. The gift of sleep (and the ability to self-soothe to sleep) is one of the best things you can give your family.
Good luck. I would start with a talk with your mom... she needs to get out of your way and let you parent your child your way. If she can't step out of the way, alternate living arrangements (if at all possible) may be necessary. Good luck... feel free to contact me if you have any questions... ____@____.com