Living with in law

Hi! So I need a bit of advice on whether or not moving in with an in law is a good idea or not. I know everyone has different experiences and opinions but here is our backstory.. My fiancé and I are having a little girl in late May. I have a 4 (almost 5 year old) from a previous relationship. We are renting our own condo/townhome (2 bedroom) and really want to buy a home. We would like to save more money to our down towards the down payment so we have decided we will rent for 1 more year and then buy. We found a lovely 3 bedroom condo/townhome in the same community we currently live in for $70 more a month which is amazing! However, my Fiancé’s mother has offered 2 bedrooms (master and an extra room for my 4 year old) for a fraction of the cost so that we can save for the year. While this will save us about $1,000 a month, I can’t help but see so many cons in this. I’m afraid this would ultimately cause problems for our family for multiple reasons.. not only does his mother live there, his sister (late 20’s), and 90 year old grandma are there as well. And while we have my 4 years old 4 out of 7 days of the week she would be with us as well. The sister consistently has friends in and out of the house (which bothers me already) and there will always be someone there between the family members so I can’t help but feel like I’d end up feeling confined to our room and out of place, which I don’t want at all. We won’t have our own home and able to live the way we have for so long, Not to mention this is my fiancé’s first child so it will be a learning experience for him and we will really need our own space to learn and grow together. But the fact that we could put away extra money monthly is amazing so it’s really hard to decide and he is pretty set on saving as much as possible to buy. Anybody have experience with living with in laws? Any advice would be so helpful, we are going to have to make a decision pretty quickly which stresses me out. Is saving money worth it?

UPDATE: we’ve decided to move into our own 3 bedroom rental for another year. While it won’t save us that much money, it doesn’t change our current finances and for piece of mind and sake of my relationship with my SO and his family, it’s worth it to me to have our own home. Thank you everyone who commented on this.