The fate of the Elliot daughters in the event of the death of Sir Walter is difficult to discuss without going beyond the first four chapters now in discussion.

However, I think I can say that the Elliot daughters are unlikely to be tossed into the hedgerows. Mary Musgrove and Lady Russell could be relied on to take them in, and it is highly possible that the marriage settlement between Sir Walter and Lady Elliot included the setting aside of some money for any children of the marriage. Lawyers at that time appeared to be happy to consider all sorts of outcomes such as wastrel husbands and selfish stepmothers in second marriages.
For example General Tilney is unable to disinherit Henry Tilney. We are told in Chapter 31 of Northanger Abbey that Of a very considerable fortune, his son was [Henry], by marriage settlements, eventually secure.