I am older than her and had kids earlier and just think I am in a good position to sort of guide her a bit. Perhaps she doesn't want that though
If my own sister had been like this, I would have wanted to take her 'guidance' and shove it down her throat.
Have a read of some of the MIL threads on here, YOU are coming across way worse than those MILs.
YOU had kids earlier, so you know everything. Well you know what? You learned, probably the hard way. Your SIL is quite capable of learning that way too.
You offered her your second hand clothes. She turned them down. So what? When you have your first child the excitement of it means that MANY parents don't want second hand. They want to CHOOSE everything, make it absolutely perfect.
You wanted to look after your DC when it was very little and she turned you down. Well maybe SHE didn't want be separated from her child. Hardly the first, nor the last mother to ever be like that.
Get over yourself!!!! IT'S NOT ABOUT YOU!!!!!!
This is about her, and how she is managing being a mother. She is allowed to do it HER way without it being a rejection of you.