700 words an hour is 11 words a minute, and it's an average figure.
I think, rather than worrying, maybe you should click your phone on to record next time you are chatting and count the words. I bet you what you like you are meeting this average and more if you are reading a few books once an hour, singing, doing things like baking/painting etc.
I am also an SLT and while your SIL's advice is grounded in terms of research in language development, it's not really her place to give it to you. That's her job, she is your family.. it's usually best if the two don't mix. It's also arguably not the whole picture, either. Language development is complex and can't really be reduced to soundbites.. most of the research we have is based on mother-child dyads in middle class homes with monolingual parents. We have good reason to believe that parental input is important and we have some clear pointers as to features of parental input that may be important but it isn't easily reducible to soundbytes e.g. "use simple sentences", "take turns" etc. It is more fluid and dynamic than that.
I also have a son who's 20 mths and here is an example of a two minute conversation earlier.
^Want to go outside? Right, we need to get your coat. Where's your coat?
(looked for coat together) - Is it under there? No honey, stop that. We're not pulling books out right now. Come here, let's put your coat on. (Ds runs into other room). Ds! Come here. Do you want to go outside? (Ds - Yes) Okay, coat on. Hands out, that's right. We need your shoes too. (Ds shuz) Where are your shoes? (Ds shuz) There they are. Foot out. Push.. push.. push.. (Ds shshshshs) Wanna close it? (Ds zhozha). You closed your shoe! (ds udda wa). The other one? Foot out. Push push push. Close it. Right, coat's on, shoes on. Mama needs her coat and shoes now. Where's that coat? Yeah, there it is. Now we need to find the key. (Ds TEA!!!) Where is it (Ds points at the shelf where it normally is) Umm, no not there. I think it's upstairs. Shall we go and look for it. Where's the key? Key, where are you? I found it! Let's go! Want to bring Wilson? Yeah? Go get him!^
This is upwards of 100 words in two minutes. Do I talk like this all day? No. Is it more a reflection of my poor organisational skills than anything? Probably. Should everyone make a meal out of going outside like I do? Definitely not. Is my child massively overdeveloped because I squeeze in so much blabber many words into a simple routine? No, he probably has abotu 60 to 70 words too.
Language development is a massive area of study, and while there are definitely indicators that point to children hearing language around them as being very important, putting a figure on it is not particularly helpful. Language transmits social, familial and cultural values as well as following a developmental trajectory. There are individual differences that may be important. There will be parents who talk to their children more or less nonstop because this reflects the culture and values of that family. In other families, independence is more prized as a value. That "exploration" may be teaching your child valuable skills in terms of persistence and problem-solving, mathematics (conservation etc) and spatial ability. If your child isn't responsive to your attempts to e.g. play blocks and do jigsaws, there's no real need to become involved. As others have said, your child will develop and it may be that RIGHT NOW developmentally, what he needs is to focus on posting or sorting or moving items between containers and language is taking a back seat to that.
Take your break while you can. Enjoy reading a book or MNing or sitting on your fat arse for a bit if you want, it won't last.. that's for sure. You are doing plenty. You do not need professional help to do better because your SIL has a job in this area. Your child is using a very average number of words for his age and you are doing LOADS.